feat: allow early muxer selection by connection encrypters

[achingbrain]

Some encrypters allow sending application protocols along with the initial handshake.

We can use these to select muxers and save the round-trips of mss.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[2color]

Would this save a roundtrip only for transports that don't have their own stream multiplexer, like TCP and WebSockets?

[achingbrain]

That's correct, yes. Transports like WebRTC and WebTransport skip the encryption/muxer part of the connection upgrade since they have their own strong opinions about how to perform these.