[VID-1412] Add WebRTC transcoder fields to the SFU protocol

protobuf/video/sfu/event/events.proto

@@ -194,6 +194,21 @@ message JoinRequest {
   repeated models.SubscribeOption preferred_subscribe_options = 10;
   repeated models.ClientCapability capabilities = 11;
   models.ParticipantSource source = 12;
+  // webrtc_transcode_mode is set by a broadcaster that publishes a single
+  // high-quality layer for server-side transcoding of the quality ladder.
+  // The SFU hides such a publisher from every subscriber that is not a
+  // WebRTC transcoder egress.
+  bool webrtc_transcode_mode = 14;
+  // transcode_of_user_id is set by a WebRTC transcoder egress and names the
+  // publisher whose stream it transcodes and republishes.
+  string transcode_of_user_id = 15;
+  // transcode_of_user_session_id narrows that to the one session of the
+  // publisher it serves. A publisher can be in a call more than once - a second
+  // tab, or a reconnect whose old session has not timed out - and each
+  // publishing session gets a transcoder of its own, so a transcoder must not
+  // take a sibling session's stream, and a session other than the one being
+  // transcoded may watch the transcoder like any other subscriber.
+  string transcode_of_user_session_id = 16;
 }
 
 message ReconnectDetails {