feat(external-api) add data-channel-open event

Signals that the bridge channel is open. It may take a few ms to get established
after the conference join, so applications might be interested in using it once
ready.

modules/API/API.js

         });
     }
 
+    /**
+     * Notify external application that the data channel has been opened.
+     *
+     * @returns {void}
+     */
+    notifyDataChannelOpened() {
+        this._sendEvent({ name: 'data-channel-opened' });
+    }
+
     /**
      * Notify external application (if API is enabled) that we are ready to be
      * closed.

modules/API/external/external_api.js

     'camera-error': 'cameraError',
     'chat-updated': 'chatUpdated',
     'content-sharing-participants-changed': 'contentSharingParticipantsChanged',
+    'data-channel-opened': 'dataChannelOpened',
     'device-list-changed': 'deviceListChanged',
     'display-name-change': 'displayNameChange',
     'email-change': 'emailChange',

react/features/external-api/middleware.js

 import {
     CONFERENCE_FAILED,
     CONFERENCE_JOINED,
+    DATA_CHANNEL_OPENED,
     KICKED_OUT
 } from '../base/conference';
 import { NOTIFY_CAMERA_ERROR, NOTIFY_MIC_ERROR } from '../base/devices';
         break;
     }
 
+    case DATA_CHANNEL_OPENED:
+        APP.API.notifyDataChannelOpened();
+        break;
+
     case DOMINANT_SPEAKER_CHANGED:
         APP.API.notifyDominantSpeakerChanged(action.participant.id);
         break;