Refine backup codec policy (#1022)

* Refine backup codec policy

Make the default behavior to be a `preferred` value that
the sfu will determine proper policy by room information.

* rename

* generated protobuf

---------

Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>

Author: cnderrauber

.changeset/rotten-experts-refuse.md

@@ -0,0 +1,6 @@
+---
+"github.com/livekit/protocol": patch
+"@livekit/protocol": patch
+---
+
+Refine backup codec policy

livekit/livekit_models.pb.go

@@ -78,10 +78,13 @@ enum ImageCodec {
 
 // Policy for publisher to handle subscribers that are unable to support the primary codec of a track
 enum BackupCodecPolicy {
-  // default behavior, regress to backup codec and all subscribers will receive the backup codec
-  REGRESSION = 0;
+  // default behavior, the track prefer to regress to backup codec and all subscribers will receive the backup codec,
+  // the sfu will try to regress codec if possible but not assured.
+  PREFER_REGRESSION = 0;
   // encoding/send the primary and backup codec simultaneously
   SIMULCAST = 1;
+  // force the track to regress to backup codec, this option can be used in video conference or the publisher has limited bandwidth/encoding power
+  REGRESSION = 2;
 }
 
 message PlayoutDelay {