Make video scalability mode configurable from peerconnection level.

This CL does not aim at cleaning up simulcast/SVC configuration, just to make it possible to set the scalability mode for AV1. Implementing a codec agnostic SVC/simulcast API is a (big) project on its own.

Change-Id: Ia88df31eb1111713e5f8832e95c8db44f92887ca

BUG: webrtc:11607
Change-Id: Ia88df31eb1111713e5f8832e95c8db44f92887ca
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/192541
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Christoffer Rodbro <crodbro@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32631}

api/video_codecs/video_encoder_config.h

@@ -64,6 +64,8 @@ struct VideoStream {
   // between multiple streams.
   absl::optional<double> bitrate_priority;
 
+  absl::optional<std::string> scalability_mode;
+
   // If this stream is enabled by the user, or not.
   bool active;
 };