Add session_id to the notifications when a client joins/leaves a music session.

src/client_container.proto

@@ -125,8 +125,9 @@ message LeaveMusicSessionAck {
 // sent by server to let the rest of the participants know a user has joined. 
 // this feels a little off; client could initiate this to other clients, right?
 message UserJoinedMusicSession {
-    optional string user_id = 1; // this is the user_id and can be used for user unicast messages
-    optional string username = 2; // meant to be a display name
+    optional string session_id = 1; // the session ID
+    optional string user_id = 2; // this is the user_id and can be used for user unicast messages
+    optional string username = 3; // meant to be a display name
 }
 
 // route_to: session