Support passing RTP extensions when writing samples.

[fancycode]

This is needed to set the sdp.SDESMidURI and sdp.SDESRTPStreamIDURI RTP header extensions when sending samples to the different layers of simulcast tracks. At least I didn't find another way to do this.

I considered reusing rtp.Extension but that doesn't expose the internal data, so additional accessors and a NewExtension(id uint8, payload []byte) *Extension function would have been required in rtp.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.20%. Comparing base (c36cb6e) to head (7466116).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3391      +/-   ##
==========================================
- Coverage   85.34%   85.20%   -0.15%     
==========================================
  Files          81       81              
  Lines        9746     9751       +5     
==========================================
- Hits         8318     8308      -10     
- Misses       1006     1018      +12     
- Partials      422      425       +3     

Flag	Coverage Δ
go	85.20% <100.00%> (-0.15%)	⬇️

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[fancycode]

Improved test coverage and rebased to current master.

Not sure about the API test failure, the new parameters to WriteSample are optional, so existing callers will still work as before.

[JoTurk]

Not sure about the API test failure, the new parameters to WriteSample are optional, so existing callers will still work as before.

it's still a breaking change, please refer to Adding to a function

does this #1841 solve your issue?

[fancycode]

it's still a breaking change, please refer to Adding to a function

Right, I didn't think about this - introduced a new function.

does this #1841 solve your issue?

Yes, copying most of the existing TrackLocalStaticSample code into my project obviously works (and this is what I'm doing right now), but I assumed this is something others might also need to send simulcast streams from files, which is the reason for my PR.