Support TrackLocal RTX

[aggresss]

Description

RetransmittedPacketsReceived/PacketsLost can be accurately measured, if use distinct SSRC to transmit RTX.

See Also: https://github.com/pion/interceptor/pull/229

Reference issue

Supports TrackLocal RTX

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 16.25000% with 67 lines in your changes missing coverage. Please review.

Project coverage is 75.92%. Comparing base (dbf2254) to head (ca3d6b4). Report is 127 commits behind head on master.

Files with missing lines	Patch %	Lines
rtpsender.go	5.08%	53 Missing and 3 partials :warning:
sdp.go	0.00%	4 Missing and 2 partials :warning:
rtpcodec.go	76.92%	2 Missing and 1 partial :warning:
settingengine.go	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2676      +/-   ##
==========================================
- Coverage   76.47%   75.92%   -0.56%     
==========================================
  Files          87       87              
  Lines        9922    10002      +80     
==========================================
+ Hits         7588     7594       +6     
- Misses       1862     1931      +69     
- Partials      472      477       +5     

Flag	Coverage Δ
go	77.39% <16.25%> (-0.62%)	:arrow_down:
wasm	64.31% <0.00%> (-0.23%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[Sean-Der]

Thank you so much for this PR @aggresss sorry I didn't merge it sooner. I was distracted by OBS and then DTLS :/

I have cherry-picked your change into https://github.com/pion/webrtc/pull/2676 and writing some tests/additional changes!