[@vercel/edge] Support overriding request headers

[nuta]

This PR adds a feature in middleware to add, modify, or delete request headers. This feature is quite useful to pass data from middleware to Serverless/Edge API routes.

New APIs

Adds a new option request.headers to the MiddlewareResponseInit parameter in NextResponse.next() and NextResponse.rewrite(). It's a Header object holding all request headers. Specifically:

interface ExtraResponseInit extends ResponseInit {
  request?: {
    headers?: Headers
  }
}

Example

// api/hello.ts
export default (req, res) => {
  const valueFromMiddleware = req.headers['x-hello-from-middleware']
  return res.send(valueFromMiddleware)
}

// middleware.ts
import { next } from '@vercel/edge'

export default function middleware(request: NextRequest) {
  // Clone request headers
  const headers = new Headers(request.headers);
  // Add a new request header
  headers.set('x-hello-from-middleware', 'foo');
  // Delete a request header from the client
  headers.delete('x-from-client');

  return next({
    request: {
      headers
    }
  });
}

New middleware headers

• x-middleware-override-headers: A comma separated list of all request header names. Headers not listed will be deleted.
• x-middleware-request-<name>: A new value for the header <name>.

Related Issues

• Next.js' implementation: Support overriding request headers in middlewares next.js#41380

📋 Checklist

Tests

• The code changed/added as part of this PR has been covered with tests
• All tests pass locally with yarn test-unit

Code Review

• This PR has a concise title and thorough description useful to a reviewer
• Issue from task tracker has a link to this PR

[Schniz]