Define SimulcastStream as an alias for SpatialLayer

Step one in making it a separate type, that will be done as a
followup, after downstream code is updated to use the new name.

Bug: webrtc:11607
Change-Id: I6fa664a0729b1cfd71b7f02b6441880beee0e741
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/262806
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36946}

modules/video_coding/utility/simulcast_test_fixture_impl.cc

@@ -188,7 +188,7 @@ void ConfigureStream(int width,
                      int min_bitrate,
                      int target_bitrate,
                      float max_framerate,
-                     SpatialLayer* stream,
+                     SimulcastStream* stream,
                      int num_temporal_layers) {
   RTC_DCHECK(stream);
   stream->width = width;