webrtc: support reading and proxying multichannel Opus (#3371)

[aler9]

Fixes #3371

TODO

• [ ] fix "frame is too big error" (inside RTP Opus encoder)
• test reading multichannel Opus with Chrome
• test reading multichannel Opus with Firefox
• test reading multichannel Opus with iOs Safari
• error message in Firefox and in all browsers that do not support this
• test proxying multichannel Opus between two MediaMTX instances

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 55.56%. Comparing base (c37e895) to head (8f10f1c).

Files	Patch %	Lines
internal/protocols/webrtc/incoming_track.go	83.33%	1 Missing ⚠️
internal/protocols/webrtc/peer_connection.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3375      +/-   ##
==========================================
+ Coverage   53.97%   55.56%   +1.59%     
==========================================
  Files         190      190              
  Lines       18879    18893      +14     
==========================================
+ Hits        10190    10498     +308     
+ Misses       7890     7586     -304     
- Partials      799      809      +10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[github-actions]

This issue is mentioned in release v1.8.3 🚀
Check out the entire changelog by clicking here