RtpTransceiverInterface: introduce HeaderExtensionsNegotiated.

This changes adds exposure of a new transceiver method for
accessing header extensions that have been negotiated, following
spec details in https://w3c.github.io/webrtc-extensions/#rtcrtptransceiver-interface.

The change contains unit tests testing the functionality.

Note: support for signalling directionality of header extensions
in the SDP isn't implemented yet.

https://chromestatus.com/feature/5680189201711104.
Intent to prototype: https://groups.google.com/a/chromium.org/g/blink-dev/c/65YdUi02yZk

Bug: chromium:1051821
Change-Id: If963beed37e96eed2dff3a2822db4e30caaea4a0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/198126
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32860}

api/rtp_transceiver_interface.cc

@@ -64,6 +64,11 @@ webrtc::RTCError RtpTransceiverInterface::SetOfferedRtpHeaderExtensions(
   return webrtc::RTCError(webrtc::RTCErrorType::UNSUPPORTED_OPERATION);
 }
 
+std::vector<RtpHeaderExtensionCapability>
+RtpTransceiverInterface::HeaderExtensionsNegotiated() const {
+  return {};
+}
+
 // TODO(bugs.webrtc.org/11839) Remove default implementations when clients
 // are updated.
 void RtpTransceiverInterface::SetDirection(