perf: lazy eval headers from the requestStore (vercel#41353)

The `ReadOnlyHeaders` class extends the `Headers` class that is provided
via a polyfill. This incurs some costs the first time it's created
because we need to load that polyfill. This saves approximately 10ms on
my machine on a cold request.

This is only useful when a user does not access headers during the
request of course and when the runtime does not support fetch naitvely.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

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

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/next/server/app-render.tsx

@@ -83,6 +83,7 @@ const INTERNAL_COOKIES_INSTANCE = Symbol('internal for cookies readonly')
 function readonlyCookiesError() {
   return new Error('ReadonlyCookies cannot be modified')
 }
+
 class ReadonlyNextCookies {
   [INTERNAL_COOKIES_INSTANCE]: NextCookies
 
@@ -1602,18 +1603,33 @@ export async function renderToHTMLOrFlight(
     (renderOpts as any).previewProps
   )
 
+  let cachedHeadersInstance: ReadonlyHeaders | undefined
+  let cachedCookiesInstance: ReadonlyNextCookies | undefined
+
   const requestStore = {
-    headers: new ReadonlyHeaders(headersWithoutFlight(req.headers)),
-    cookies: new ReadonlyNextCookies({
-      headers: {
-        get: (key) => {
-          if (key !== 'cookie') {
-            throw new Error('Only cookie header is supported')
-          }
-          return req.headers.cookie
-        },
-      },
-    }),
+    get headers() {
+      if (!cachedHeadersInstance) {
+        cachedHeadersInstance = new ReadonlyHeaders(
+          headersWithoutFlight(req.headers)
+        )
+      }
+      return cachedHeadersInstance
+    },
+    get cookies() {
+      if (!cachedCookiesInstance) {
+        cachedCookiesInstance = new ReadonlyNextCookies({
+          headers: {
+            get: (key) => {
+              if (key !== 'cookie') {
+                throw new Error('Only cookie header is supported')
+              }
+              return req.headers.cookie
+            },
+          },
+        })
+      }
+      return cachedCookiesInstance
+    },
     previewData,
   }