Support turn.urls with TURN service

For backwards compatibility we return the first in list TURN server directly. This allows clients which do not implement any selection still use TURN and lets the TURN service control which one it should be (first in list is used).

go/channelling/turnservice_manager.go

@@ -97,6 +97,13 @@ func (mgr *turnServiceManager) turnData(credentials *turnservicecli.CachedCreden
 			turn.Servers = credentials.Turn.Servers
 			turn.Ttl = int(ttl)
 			turn.GeoURI = credentials.Turn.GeoURI
+
+			if len(turn.Servers) > 0 {
+				// For backwards compatibility with clients which do not
+				// understand turn.Servers, directly deliver the first TURN
+				// server zone URNs.
+				turn.Urls = turn.Servers[0].URNs
+			}
 		}
 	}