feat(ext/http): websockets over h2

[AaronO]

A first pass at supporting WebSockets over http/2 server-side.

A few notes & minor pending items:

• Unclear how we should expose pseudo-headers like :protocol to userspace, I don't think there's precedent, since other pseudo-headers like :method, :path, :authority are mostly handled as one-offs and map to h1 semantics
• We don't support ws-over-h2 client-side, making it a little more tedious to test
• We could do client-side ws-over-h2, but it requires more work and isn't something I need at present

[aapoalas]

Sorry, can you rebase this? Thank you <3

[ry]

closing this because it's old