WebTransport support

[FZambia]

An issue to track WebTransport support:

• Make a POC based on quic-go Webtransport pull request
• Make sure Disconnect logic matches ours (i.e. WT can be closed with code and string reason)
• Add StreamCommandDecoder which accepts io.Reader and exposes Commands
• Modify DataEncoder to always append \n to JSON, for single WebSocket messages we have fast path which does not use DataEncoder – so for single messages \n won't be included into payload sent.
• Expose handleCommand to Client's public API. Maybe remove Client.Handle and move command decoding to Transport.
• With the base above introduce an example with NewWebtransportHandler and WebtransportConfig which uses quic-go for WT stuff.

[FZambia]

We experimentally support WebTransport in Centrifugo now. The code is here: https://github.com/centrifugal/centrifugo/tree/master/internal/wt.