Add GetRtpExtensionMap to ReceiveStream and remove GetRtpExtensions.

GetRtpExtensions() is still used in one corner case for audio receive
streams, so GetRtpExtensions has migrated to AudioReceiveStream.

Updated FlexfecReceiveStream config management (incl. pass by value) and
now store an RtpHeaderExtensionMap in FlexfecReceiveStreamImpl.

Call GetRtpExtensionMap() from call.cc instead of constructing one on
the fly for each rtp packet (for video packets at least).

Bug: webrtc:11993
Change-Id: Id90ec5d43ea368f58edd6f17cb39d8c54aec641f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261800
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36839}
diff --git a/call/call.h b/call/call.h
index e4652d2..9d6d4ee 100644
--- a/call/call.h
+++ b/call/call.h
@@ -127,7 +127,7 @@
   // protected by a FlexfecReceiveStream, the latter should be created before
   // the former.
   virtual FlexfecReceiveStream* CreateFlexfecReceiveStream(
-      const FlexfecReceiveStream::Config& config) = 0;
+      const FlexfecReceiveStream::Config config) = 0;
   virtual void DestroyFlexfecReceiveStream(
       FlexfecReceiveStream* receive_stream) = 0;