Add MID sending to RTPSender

This CL adds the ability to configure RTPSender to include the
MID header extension when sending packets. The MID will be
included on every packet at the start of the stream until an RTCP
acknoledgment is received for that SSRC at which point it will
stop being included. The MID will be included on regular RTP
streams as well as RTX streams.

Bug: webrtc:4050
Change-Id: Ie27ebee1cd00a67f2b931f5363788f523e3e684f
Reviewed-on: https://webrtc-review.googlesource.com/60582
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22574}

modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -13,6 +13,7 @@
 
 #include <memory>
 #include <set>
+#include <string>
 #include <utility>
 #include <vector>
 
@@ -86,6 +87,8 @@ class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::ModuleRtpRtcp {
   // Configure SSRC, default is a random number.
   void SetSSRC(uint32_t ssrc) override;
 
+  void SetMid(const std::string& mid) override;
+
   void SetCsrcs(const std::vector<uint32_t>& csrcs) override;
 
   RTCPSender::FeedbackState GetFeedbackState();