Add temporary method SimulcastStream::SetNumberOfTemporalLayers

Similar to https://webrtc-review.googlesource.com/c/src/+/267843, it
turns out we need the setter method too to transition downstream code.

Bug: webrtc:11607
Change-Id: I50df5e9c5d9301717d527089de61fcf783267ee1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/268760
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37545}

api/video_codecs/simulcast_stream.h

@@ -22,6 +22,7 @@ struct SimulcastStream {
   // setting to ScalabilityMode.
   unsigned char GetNumberOfTemporalLayers() const;
   ScalabilityMode GetScalabilityMode() const;
+  void SetNumberOfTemporalLayers(unsigned char n);
 
   int width = 0;
   int height = 0;