Handle scalability mode in QueryCodecSupport

All valid scalability modes should be supported by the builtin
software decoder/encoder.

Bug: chromium:1187565
Change-Id: If66105d210d5055019f35dae2f80a18ad4a70cdd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/222642
Commit-Queue: Johannes Kron <kron@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34998}

modules/video_coding/codecs/av1/libaom_av1_encoder.cc

@@ -806,4 +806,11 @@ std::unique_ptr<VideoEncoder> CreateLibaomAv1Encoder() {
   return std::make_unique<LibaomAv1Encoder>();
 }
 
+bool LibaomAv1EncoderSupportsScalabilityMode(
+    absl::string_view scalability_mode) {
+  // For AV1, the scalability mode is supported if we can create the scalability
+  // structure.
+  return ScalabilityStructureConfig(scalability_mode) != absl::nullopt;
+}
+
 }  // namespace webrtc