feat: Add option to force pc to use turn relay candidates.

modules/RTC/RTC.js

             iceConfig.sdpSemantics = 'plan-b';
         }
 
+        if (options.forceTurnRelay) {
+            iceConfig.iceTransportPolicy = 'relay';
+        }
+
         // Set the RTCBundlePolicy to max-bundle so that only one set of ice candidates is generated.
         // The default policy generates separate ice candidates for audio and video connections.
         // This change is necessary for Unified plan to work properly on Chrome and Safari.

modules/xmpp/JingleSessionPC.js

         pcOptions.capScreenshareBitrate = false;
         pcOptions.enableInsertableStreams = options.enableInsertableStreams;
         pcOptions.videoQuality = options.videoQuality;
+        pcOptions.forceTurnRelay = options.forceTurnRelay;
 
         // codec preference options for jvb connection.
         if (pcOptions.videoQuality) {