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

[aler9]

Fixes #3371

TODO

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

[codecov[bot]]

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 :warning:
internal/protocols/webrtc/peer_connection.go	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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 ```

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[github-actions[bot]]

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