net/http: support HTTP/3

[johanbrandhorst]

I searched for an issue to track progress of implementing HTTP/3 in the standard library but couldn't find one, so I'm opening this new issue to track this (presumably inevitable) effort.

HTTP/3 is still an IETF draft, so there's not too much point starting work on this yet. The relevant spec is (currently) available here: https://quicwg.org/base-drafts/draft-ietf-quic-http.html.

Some questions we may want to answer before any work starts:

1. Should work on this start in one of the x repos, like HTTP/2 did?
2. What if any existing libraries can be used as a base for the new HTTP/3 work?

Of note is an existing attempt at implementing QUIC and HTTP/3 in pure go: https://github.com/lucas-clemente/quic-go.