Pass estimator options through GCC

[jayli-nuro]

The GCC option constructs the estimator itself, so a caller can only influence it through the two bitrate arguments:

opts := []gcc.Option{gcc.SendSideBWEInitialBitrate(initialBitrate)}
if maxBitrate > 0 {
    opts = append(opts, gcc.SendSideBWEMaxBitrate(maxBitrate))
}

return gcc.NewSendSideBWE(opts...)

Every other gcc.Option — the pacer, and anything added to that package later — is unreachable. Configuring the estimator at all currently means editing this package.

This accepts a variadic ...gcc.Option on GCC, setupGCC and newGCCFactory, and appends it after the bitrate options:

opts = append(opts, extra...)

Appending last is deliberate, so a caller can also override the two bitrate options rather than only add to them.

Backward compatible. Adding a variadic parameter does not change existing call sites, so the two-argument callers in examples/ and the tests are untouched. TestGCCRemainsCallableWithTwoArguments pins that.

Three tests: that an option reaches NewSendSideBWE, that a later option overrides the initialBitrate argument, and that the two-argument form still compiles and runs. Removing the append fails the first two.

gofmt, go vet and go test ./sender/ are green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.70%. Comparing base (76ac6d1) to head (70e69ea).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
+ Coverage   52.59%   52.70%   +0.11%     
==========================================
  Files          21       21              
  Lines        2181     2182       +1     
==========================================
+ Hits         1147     1150       +3     
+ Misses        947      946       -1     
+ Partials       87       86       -1     

Flag	Coverage Δ
go	52.70% <100.00%> (+0.11%)	⬆️

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