Remove packets from RtpPacketHistory if acked via TransportFeedback

If the receiver has indicated that a packet has been received, via a
TransportFeedback RTCP message, it is safe to remove it from the
RtpPacketHistory as we can be sure it won't be needed anymore.
This will reduce memory usage, reduce the risk of overflow in the
history at very high bitrates, and hopefully make payload based padding
a little more useful.

Bug: webrtc:8975
Change-Id: I703a353252943f63d7d6edda68f03bc482633fd6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/133028
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27745}

modules/rtp_rtcp/include/rtp_rtcp.h

@@ -275,6 +275,11 @@ class RtpRtcp : public Module, public RtcpFeedbackSenderInterface {
   virtual StreamDataCountersCallback* GetSendChannelRtpStatisticsCallback()
       const = 0;
 
+  // Returns a pointer to an observer that handles information about packets
+  // that have been received by the remote end, or nullptr if not applicable.
+  virtual AcknowledgedPacketsObserver* GetAcknowledgedPacketsObserver()
+      const = 0;
+
   // **************************************************************************
   // RTCP
   // **************************************************************************