quic: http/3 datagrams partially diverge from spec

[martenrichter]

Http datagrams are according to spec composed as:

HTTP/3 Datagram {
  Quarter Stream ID (i),
  HTTP Datagram Payload (..),
}

and beside this a normal quic datagram.
If I understand the current implementation correctly, the quarter stream id of the bidirectional stream is not included,
and just unfiltered quicdatagrams are sent.
Of course, it is not difficult to sort this issue out on js side, but may be users may stumble upon this.
@jasnell @pimterry what do you think?

Implementing this would not be really hard.

[pimterry]

Yep, I'm on it, I've got a working version already. It's one place where the HTTP vs QUIC conflict really bites since both APIs support the same thing in explicitly incompatible ways (session-linked vs stream-linked).

[martenrichter]

Well the users can also implement it manually:
fails-components/webtransport@1fbd2f1
But the required additional copy and parsing on the JS side is not really a good idea.

[jasnell]

worth doing right built-in.