HTTP 103 Early Hints

https://evertpot.com/http/103-early-hints

Request from: https://github.com/mcollina/undici/issues/179

> Related topic; how would HTTP 103 Early Hints play out? It would be very nice to have a way to get the early hints data in a way before the header and body.
> 
> I have a real use case for 103.

@trygve-lie