Implement the encoding RtpParameter scaleResolutionDownBy

Support varies by codec, especially in the simulcast case, but using
the EncoderSimulcastProxy codec should fix this.

Bug: webrtc:10069
Change-Id: Idb6a5f400ffda1cdb139004f540961a9cf85d224
Reviewed-on: https://webrtc-review.googlesource.com/c/119400
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26449}

api/video_codecs/video_encoder_config.cc

@@ -22,6 +22,7 @@ VideoStream::VideoStream()
       min_bitrate_bps(-1),
       target_bitrate_bps(-1),
       max_bitrate_bps(-1),
+      scale_resolution_down_by(-1.),
       max_qp(-1),
       active(true) {}
 VideoStream::VideoStream(const VideoStream& other) = default;