[dev] Support request headers override in middleware (#8752)

Implements request headers override in middlewares.

#### 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

- #8724: Add helper functions for non-Next.js middlewares
- https://github.com/vercel/next.js/pull/41380: Next.js' implementation

### 📋 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

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- [ ] Issue from task tracker has a link to this PR

packages/cli/src/util/dev/server.ts

@@ -87,7 +87,7 @@ import {
 } from './types';
 import { ProjectSettings } from '../../types';
 import { treeKill } from '../tree-kill';
-import { nodeHeadersToFetchHeaders } from './headers';
+import { applyOverriddenHeaders, nodeHeadersToFetchHeaders } from './headers';
 import { formatQueryString, parseQueryString } from './parse-query-string';
 import {
   errorToString,
@@ -1472,6 +1472,9 @@ export default class DevServer {
             'content-length',
             'transfer-encoding',
           ]);
+
+          applyOverriddenHeaders(req.headers, middlewareRes.headers);
+
           for (const [name, value] of middlewareRes.headers) {
             if (name === 'x-middleware-next') {
               shouldContinue = value === '1';