fix tuic implementation

[dyhkwong]

• headerSize of udpMessage is incorrect.
• Server-to-client CommandHeartbeat does not exist. The specification and the original server implementation don't have this, and the original client implementation warns about this unknown command.
• Let the client ignore unknown commands. This is the behavior of the original client implementation, and it is also needed for those sing-quic servers sending CommandHeartbeat to clients.
• Client should not loopMessages if udpStream is enabled, and should not loopUniStreams if udpStream is disabled.

P.S. There is a Hysteria2 HTTP/3 issue that http3.ConfigureTLSConfig returns a new tls.Config rather than modifying the given tlsConfig.

sing-quic/quic.go

Line 117 in 00a5561

http3.ConfigureTLSConfig(tlsConfig)

Fixing this may need new interfaces.