Add backchannel support for rtsp server

[seydx]

Currently, there is no way for clients to send audio data to go2rtc via RTSP backchannel, as the RTSP server lacks a backchannel implementation.

This PR adds a backchannel to the stream (when backchannel=1) and thus allows clients to send audio data to go2rtc.

Tested streams so far:

• webrtc:ws://
• tapo://
• rtsp:// with backchannel

I think other streams with backchannel should also work

[felipecrs]

@seydx, out of curiosity, what use case this enables? Connecting a go2rtc server to another?

[seydx]

@seydx, out of curiosity, what use case this enables? Connecting a go2rtc server to another?

This implementation allows me to send audio data directly to go2rtc using my RTSP client. The data is sent interleaved over the connected socket

Since go2rtc offers two-way audio for some cameras, e.g. tapo etc, the rtsp client can connect to the go2rtc rtsp link of the tapo camera and send audio data directly

This functionality is natively available in WebRTC, where you can send audio directly from the browser to the camera. With this implementation, virtually anyone can do it, regardless of the programming language​​​​​​​​​​​​​​​​ and environment, since rtsp ist just a tcp connection