CORS support

If a request comes from a script running under a different origin (for our purposes here, a [tuple](https://html.spec.whatwg.org/multipage/origin.html#concept-origin-tuple) that includes a host and port) from the origin that is hosting a function app, the request will fail due to lack of cross-origin resource sharing ([CORS protocol](https://fetch.spec.whatwg.org/#http-cors-protocol)) support.

For background, browsers restrict cross-origin HTTP requests initiated from scripts for security purposes, so if a browser runs, for example, a Flutter app loaded from a different origin than the origin hosting, for example, a Dart function app, the request will fail if the function app doesn't support CORS.

As part of supporting CORS, a function app must be able to handle the [CORS preflight](https://fetch.spec.whatwg.org/#cors-preflight-request), which is sent as an HTTP [OPTIONS](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/OPTIONS) request.

What would be useful for this situation is to add CORS support to the framework that would allow specifying a CORS policy, possible as metadata annotation or as an API, for setting a policy. For inspiration, see the Node.js [cors package](https://expressjs.com/en/resources/middleware/cors.html).

References:
* https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
* https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/OPTIONS
* https://fetch.spec.whatwg.org/#http-cors-protocol
* https://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html