Add the public API for send side BWE

[mengelbart]

The current API requires users to call OnAck and OnLoss for each packet in a report, then call OnFeedback. An alternative would be to have only a single method accepting a report data structure so the controller can read (and sort) it internally. The downside would be that we need a common data structure for this, which would force users to build it manually and require additional loops (the client loops through his report to build the structure, and we loop through it to consume it). That may be fine for use within pion, but it makes it harder to use the algorithm elsewhere. Any opinions?

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.81%. Comparing base (3d39820) to head (4d3b0e9).

Files with missing lines	Patch %	Lines
gcc/send_side_bwe.go	88.88%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   99.09%   97.81%   -1.28%     
==========================================
  Files           9       10       +1     
  Lines         441      504      +63     
==========================================
+ Hits          437      493      +56     
- Misses          2        6       +4     
- Partials        2        5       +3     

Flag	Coverage Δ
go	97.81% <88.88%> (-1.28%)	⬇️

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