Add Estimator accessor to cc.Interceptor

Currently, the congestion control interceptor communicates
the estimator to the application by using a callback, even
though creating the interceptor is synchronous operation.
Allow applications to avoid the callback by adding  a public
accessor.

Author: jech

pkg/cc/interceptor.go

@@ -56,6 +56,7 @@ func NewInterceptor(factory BandwidthEstimatorFactory, opts ...Option) (*Interce
 
 // OnNewPeerConnection sets a callback that is called when a new CC interceptor
 // is created.
+// Deprecated: use [Estimator] instead.
 func (f *InterceptorFactory) OnNewPeerConnection(cb NewPeerConnectionCallback) {
 	f.addPeerConnection = cb
 }
@@ -132,3 +133,7 @@ func (c *Interceptor) BindLocalStream(info *interceptor.StreamInfo, writer inter
 func (c *Interceptor) Close() error {
 	return c.estimator.Close()
 }
+
+func (c *Interceptor) Estimator() BandwidthEstimator {
+	return c.estimator
+}