add Cloudflare Turnstile example (#41283)

## Description
close #41110 

## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm lint`
- [x] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

examples/cloudflare-turnstile/.env.local.example

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+# Public Environment variables that can be used in the browser.
+NEXT_PUBLIC_CLOUDFLARE_TURNSTILE_SITE_KEY=
+
+# Secret environment variables only available to Node.js
+CLOUDFLARE_TURNSTILE_SECRET_KEY=

examples/cloudflare-turnstile/.gitignore

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+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+/.pnp
+.pnp.js
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+.pnpm-debug.log*
+
+# local env files
+.env*.local
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo
+next-env.d.ts

examples/cloudflare-turnstile/README.md

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+# Example with Cloudflare Turnstile
+
+[Turnstile](https://developers.cloudflare.com/turnstile/) is Cloudflare’s smart CAPTCHA alternative. It can be embedded into any website without sending traffic through Cloudflare and works without showing visitors a CAPTCHA.
+
+This example shows how you can use **Cloudflare Turnstile** with your Next.js project. You can see a [live version here](https://with-cloudflare-turnstile.vercel.app/).
+
+## Deploy your own
+
+Deploy the example using [Vercel](https://vercel.com?utm_source=github&utm_medium=readme&utm_campaign=next-example).
+
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/new/git/external?repository-url=https://github.com/vercel/next.js/tree/canary/examples/cloudflare-turnstile&project-name=cloudflare-turnstile&repository-name=cloudflare-turnstile)
+
+**Important**: When you import your project on Vercel, make sure to click on **Environment Variable**s and set them to match your .env.local file.
+
+## How to use
+
+Execute [`create-next-app`](https://github.com/vercel/next.js/tree/canary/packages/create-next-app) with [npm](https://docs.npmjs.com/cli/init), [Yarn](https://yarnpkg.com/lang/en/docs/cli/create/), or [pnpm](https://pnpm.io) to bootstrap the example:
+
+```bash
+npx create-next-app --example cloudflare-turnstile cloudflare-turnstile-app
+```
+
+```bash
+yarn create next-app --example cloudflare-turnstile cloudflare-turnstile-app
+```
+
+```bash
+pnpm create next-app --example cloudflare-turnstile cloudflare-turnstile-app
+```
+
+Deploy it to the cloud with [Vercel](https://vercel.com/new?utm_source=github&utm_medium=readme&utm_campaign=next-example) ([Documentation](https://nextjs.org/docs/deployment)).
+
+## Configuring Cloudflare Turnstile
+
+### Get a sitekey and secret key
+
+1. Go to the [Cloudflare dashboard](https://dash.cloudflare.com/?to=/:account/turnstile) and select your account.
+2. Go to Turnstile.
+3. Select Add a site and fill out the form.
+4. Copy your **Site Key** and **Secret Key**.
+
+### Set up environment variables
+
+To connect the app with Cloudflare Turnstile, you'll need to add the settings from your Cloudflare dashboard as environment variables
+
+Copy the .env.local.example file in this directory to .env.local.
+
+```bash
+cp .env.local.example .env.local
+```
+
+Then, open .env.local and fill these environment variables:
+
+- `NEXT_PUBLIC_CLOUDFLARE_TURNSTILE_SITE_KEY`
+- `CLOUDFLARE_TURNSTILE_SECRET_KEY`

examples/cloudflare-turnstile/app.css

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+html,
+body {
+  display: grid;
+  place-content: center;
+  margin: 0;
+  height: 100%;
+}
+
+main {
+  height: 50vh;
+  width: 300px;
+  text-align: center;
+}
+
+button[type='submit'] {
+  cursor: pointer;
+  width: 100%;
+  outline: none;
+  border: none;
+  padding: 0.5rem 1rem;
+  margin-top: 1rem;
+  border-radius: 0.25rem;
+  background: #0d6efd;
+  font-size: 1.25rem;
+  color: #ffffff;
+}
+
+.checkbox {
+  min-height: 70px;
+}

examples/cloudflare-turnstile/next.config.js

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+module.exports = {
+  async rewrites() {
+    return [
+      {
+        source: '/',
+        destination: '/implicit',
+      },
+    ]
+  },
+}

examples/cloudflare-turnstile/package.json

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+{
+  "private": true,
+  "scripts": {
+    "dev": "next",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "next": "latest",
+    "react": "^18.2.0",
+    "react-dom": "^18.2.0"
+  },
+  "devDependencies": {
+    "@types/node": "^18.0.0",
+    "@types/react": "^18.0.14",
+    "@types/react-dom": "^18.0.5",
+    "typescript": "^4.7.4"
+  }
+}

examples/cloudflare-turnstile/pages/_app.tsx

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+import type { AppProps } from 'next/app'
+import '../app.css'
+
+export default function App({ Component, pageProps }: AppProps) {
+  return <Component {...pageProps} />
+}

examples/cloudflare-turnstile/pages/api/handler.ts

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+import type { NextApiRequest, NextApiResponse } from 'next'
+
+export default async function Handler(
+  req: NextApiRequest,
+  res: NextApiResponse
+) {
+  const form = new URLSearchParams()
+  form.append('secret', process.env.CLOUDFLARE_TURNSTILE_SECRET_KEY)
+  form.append('response', req.body['cf-turnstile-response'])
+  form.append('remoteip', req.headers['x-forwarded-for'] as string)
+
+  const result = await fetch(
+    'https://challenges.cloudflare.com/turnstile/v0/siteverify',
+    { method: 'POST', body: form }
+  )
+  const json = await result.json()
+  res.status(result.status).json(json)
+}

examples/cloudflare-turnstile/pages/explicit.tsx

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+import Script from 'next/script'
+
+type RenderParameters = {
+  sitekey: string
+  theme?: 'light' | 'dark'
+  callback?(token: string): void
+}
+
+declare global {
+  interface Window {
+    onloadTurnstileCallback(): void
+    turnstile: {
+      render(container: string | HTMLElement, params: RenderParameters): void
+    }
+  }
+}
+
+export default function ExplicitRender() {
+  return (
+    <main>
+      <Script id="cf-turnstile-callback">
+        {`window.onloadTurnstileCallback = function () {
+          window.turnstile.render('#my-widget', {
+            sitekey: '${process.env.NEXT_PUBLIC_CLOUDFLARE_TURNSTILE_SITE_KEY}',
+          })
+        }`}
+      </Script>
+      <Script
+        src="https://challenges.cloudflare.com/turnstile/v0/api.js?onload=onloadTurnstileCallback"
+        async={true}
+        defer={true}
+      />
+      <form method="POST" action="/api/handler">
+        <h2>Dummy Login Demo</h2>
+        <div id="my-widget" className="checkbox" />
+        <button type="submit">Sign in</button>
+        <p>
+          Go to the <a href="/implicit">implicit render demo</a>
+        </p>
+      </form>
+    </main>
+  )
+}

examples/cloudflare-turnstile/pages/implicit.tsx

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+import Script from 'next/script'
+
+export default function ImplicitRender() {
+  return (
+    <main>
+      <Script
+        src="https://challenges.cloudflare.com/turnstile/v0/api.js"
+        async={true}
+        defer={true}
+      />
+      <form method="POST" action="/api/handler">
+        <h2>Dummy Login Demo</h2>
+        <div
+          className="cf-turnstile checkbox"
+          data-sitekey={process.env.NEXT_PUBLIC_CLOUDFLARE_TURNSTILE_SITE_KEY}
+        />
+        <button type="submit">Sign in</button>
+        <p>
+          Go to the <a href="/explicit">explicit render demo</a>
+        </p>
+      </form>
+    </main>
+  )
+}

examples/cloudflare-turnstile/tsconfig.json

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+{
+  "compilerOptions": {
+    "target": "es5",
+    "lib": ["dom", "dom.iterable", "esnext"],
+    "allowJs": true,
+    "skipLibCheck": true,
+    "strict": false,
+    "forceConsistentCasingInFileNames": true,
+    "noEmit": true,
+    "esModuleInterop": true,
+    "module": "esnext",
+    "moduleResolution": "node",
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "jsx": "preserve",
+    "incremental": true
+  },
+  "include": ["next-env.d.ts", "**/*.ts", "**/*.tsx"],
+  "exclude": ["node_modules"]
+}