Add missing a=ssrc lines for RTX/FEC

[Sean-Der]

Pion extracts them from ssrc-group line so this didn't failed Pion<->Pion. Chrome/FireFox uses the dedicated a=ssrc lines and our Offer/Answer would break negotation

Fixes #2922

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.00%. Comparing base (9d6c27f) to head (aa0d9ab).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
sdp.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2926      +/-   ##
==========================================
- Coverage   79.01%   79.00%   -0.02%     
==========================================
  Files          89       89              
  Lines        8564     8568       +4     
==========================================
+ Hits         6767     6769       +2     
- Misses       1310     1311       +1     
- Partials      487      488       +1     

Flag	Coverage Δ
go	80.55% <50.00%> (-0.02%)	⬇️
wasm	64.47% <ø> (ø)

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