103 Early Hints

Something to consider as a future feature would be support for 103 Early Hints:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/103

This would probably require a new method on Response or something.

One thing to note though, is that this is recommended only for HTTP/2. So, we should probably find a way to detect if the proxy server is using an HTTP/2 connection with the client, if possible, and skip the output if not. If the user is not using a proxy server, it'll never send the response as we don't have HTTP/2 support.

It looks like Via may contain this information:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Via