Add support for RTX via distinct SSRC

[k0nserv]

We don't support RTP retransmissions(RTX) when the retransmitted packets are sent over a separate RTP stream(with a distinct SSRC). These packets are simply black holed at the moment.

For now RTX based on NACKs emitted by webrtc-rs work, but the RTX packets will be sent within the established RTP stream.

Related issues in Pion:

• Unable to handle NACK that uses distinct SSRC pion/webrtc#1675
• How can I receive or deal the RTX packet? pion/webrtc#2272

[k0nserv]

I'm removing the default registration of RTX codecs in #294