[@vercel/edge] Support overriding request headers (#8724)

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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`](https://developer.mozilla.org/en-US/docs/Web/API/Headers)
object holding *all* request headers. Specifically:

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

### Example

```ts
// 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: https://github.com/vercel/next.js/pull/41380

### 📋 Checklist

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#### 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

Co-authored-by: Gal Schlezinger <gal@spitfire.co.il>

packages/edge/docs/interfaces/ExtraResponseInit.md

@@ -11,6 +11,7 @@
 ### Properties
 
 - [headers](ExtraResponseInit.md#headers)
+- [request](ExtraResponseInit.md#request)
 - [status](ExtraResponseInit.md#status)
 - [statusText](ExtraResponseInit.md#statustext)
 
@@ -25,7 +26,19 @@ along with the response headers from the origin.
 
 #### Defined in
 
-[src/middleware-helpers.ts:6](https://github.com/vercel/vercel/blob/main/packages/edge/src/middleware-helpers.ts#L6)
+[src/middleware-helpers.ts:31](https://github.com/vercel/vercel/blob/main/packages/edge/src/middleware-helpers.ts#L31)
+
+---
+
+### request
+
+• `Optional` **request**: [`ModifiedRequest`](ModifiedRequest.md)
+
+Fields to rewrite for the upstream request.
+
+#### Defined in
+
+[src/middleware-helpers.ts:35](https://github.com/vercel/vercel/blob/main/packages/edge/src/middleware-helpers.ts#L35)
 
 ---
 

packages/edge/docs/interfaces/ModifiedRequest.md

@@ -0,0 +1,38 @@
+# Interface: ModifiedRequest
+
+## Table of contents
+
+### Properties
+
+- [headers](ModifiedRequest.md#headers)
+
+## Properties
+
+### headers
+
+• `Optional` **headers**: `Headers`
+
+If set, overwrites the incoming headers to the origin request.
+
+This is useful when you want to pass data between a Middleware and a
+Serverless or Edge Function.
+
+**`Example`**
+
+<caption>Add a `x-user-id` header and remove the `Authorization` header</caption>
+
+```ts
+import { rewrite } from '@vercel/edge';
+export default async function middleware(request: Request): Promise<Response> {
+  const newHeaders = new Headers(request.headers);
+  newHeaders.set('x-user-id', 'user_123');
+  newHeaders.delete('authorization');
+  return rewrite(request.url, {
+    request: { headers: newHeaders },
+  });
+}
+```
+
+#### Defined in
+
+[src/middleware-helpers.ts:23](https://github.com/vercel/vercel/blob/main/packages/edge/src/middleware-helpers.ts#L23)