Implement RTCOutboundRtpStreamStats.targetBitrate for audio.

Spec: https://w3c.github.io/webrtc-stats/#dom-rtcoutboundrtpstreamstats-targetbitrate

Bug: webrtc:13377
Change-Id: I98dd263e0b9d6e2ca94969d2a91857b14cd65f70
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/237402
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Jakob Ivarsson <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35337}

modules/audio_coding/include/audio_coding_module.h

@@ -237,6 +237,8 @@ class AudioCodingModule {
       NetworkStatistics* network_statistics) = 0;
 
   virtual ANAStats GetANAStats() const = 0;
+
+  virtual int GetTargetBitrate() const = 0;
 };
 
 }  // namespace webrtc