FEC interceptor enhancements

[aalekseevx]

Description

1. Pass in correct payload type and ssrc (don't use the video track's payload type and ssrc
2. Store only main ssrc packets in buffer
3. Allow custom FEC encoders
4. Add options for NumMediaPackets and NumFECPackets
5. Allow multiple RTP streams with FEC support
6. Encode FEC only if negotiated

Reference issue

Fixes #332

[codecov]

Codecov Report

Attention: Patch coverage is 82.43243% with 13 lines in your changes missing coverage. Please review.

Project coverage is 78.44%. Comparing base (8d3fc6d) to head (8d033dc).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/flexfec/encoder_interceptor.go	75.92%	10 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #333      +/-   ##
==========================================
+ Coverage   77.65%   78.44%   +0.78%     
==========================================
  Files          80       81       +1     
  Lines        5080     5130      +50     
==========================================
+ Hits         3945     4024      +79     
+ Misses        960      929      -31     
- Partials      175      177       +2     

Flag	Coverage Δ
go	78.44% <82.43%> (+0.78%)	⬆️
wasm	76.08% <82.43%> (+0.74%)	⬆️

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[JoTurk]

I can confirm it's working, This is great, thank you so much <3

[aalekseevx]

I think we shouldn't keep FlexEncoder20 in the public API until it's working, so compatibility check is red. @joeturki, do you think it's ok to bypass the rules here?

[JoTurk]

I think we shouldn't keep FlexEncoder20 in the public API until it's working, so compatibility check is red. @joeturki, do you think it's ok to bypass the rules here?

In one hand interceptor is version 0, so breaking changes like this is expected and this removes already broken API, so it's probably not used on production anywhere, on the other hand I don't see a problem of keeping until it's fixed.

@Sean-Der what do you think?

[3DRX]

This looks great!

[aalekseevx]

@joeturki, I have returned the broken API – I'm fine with that