Add support for scalability modes L2T3 and S2T3

Bug: webrtc:11607
Change-Id: I1d0bd171564d2852f2f6ee2bbee26c7a1c0e1c3f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/267103
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37389}

api/video_codecs/scalability_mode.h

@@ -44,6 +44,7 @@ enum class ScalabilityMode {
   kL3T3h,
   kL3T3_KEY,
   kS2T1,
+  kS2T3,
   kS3T3,
 };