feat(external_api): Command to set participant volume

modules/API/API.js

 } from '../../react/features/device-selection/functions';
 import { isEnabled as isDropboxEnabled } from '../../react/features/dropbox';
 import { toggleE2EE } from '../../react/features/e2ee/actions';
+import { setVolume } from '../../react/features/filmstrip';
 import { invite } from '../../react/features/invite';
 import {
     selectParticipantInLargeVideo
             sendAnalytics(createApiEvent('largevideo.participant.set'));
             APP.store.dispatch(selectParticipantInLargeVideo(participantId));
         },
+        'set-participant-volume': (participantId, volume) => {
+            APP.store.dispatch(setVolume(participantId, volume));
+        },
         'subject': subject => {
             sendAnalytics(createApiEvent('subject.changed'));
             APP.store.dispatch(setSubject(subject));

modules/API/external/external_api.js

     sendTones: 'send-tones',
     setFollowMe: 'set-follow-me',
     setLargeVideoParticipant: 'set-large-video-participant',
+    setParticipantVolume: 'set-participant-volume',
     setTileView: 'set-tile-view',
     setVideoQuality: 'set-video-quality',
     startRecording: 'start-recording',