websocket over http2

[KhafraDev]

Refs: #2061 (comment)

[metcoder95]

Doing a quick check, the feature might not be that hard to add.
We basically have to tweak the first request to conform the WebSocket negotiation spec.

So far, the overall implementation should not need changes; rather just writeH2.
Fetch already provides the mode through upgrade here: https://github.com/nodejs/undici/blob/ac30400a191a78c6148c50e7ff70390a4ea93f90/lib/fetch/index.js#L1963C9-L1963C16

It is a matter of writeH2 to catch it and attach it with the correct semantics at the headers if the connection is happening through h2, and call onUpgrade if everything went well accordingly.
As fetch uses the socket directly, it should be even less complicated. But we would need to play with it to see if it works.

[baybal]

FYI, here is the RFC for web socket over http 2.0 https://www.rfc-editor.org/rfc/rfc8441