feat(iFrame): Add method for pinning a participant on stage view

modules/API/API.js

 } from '../../react/features/base/conference';
 import { parseJWTFromURLParams } from '../../react/features/base/jwt';
 import { JitsiRecordingConstants } from '../../react/features/base/lib-jitsi-meet';
+import { pinParticipant } from '../../react/features/base/participants';
 import {
     processExternalDeviceRequest
 } from '../../react/features/device-selection/functions';
                 ));
             }
         },
+        'pin-participant': id => {
+            logger.debug('Pin participant command received');
+            sendAnalytics(createApiEvent('participant.pinned'));
+            APP.store.dispatch(pinParticipant(id));
+        },
         'proxy-connection-event': event => {
             APP.conference.onProxyConnectionEvent(event);
         },

modules/API/external/external_api.js

     hangup: 'video-hangup',
     muteEveryone: 'mute-everyone',
     password: 'password',
+    pinParticipant: 'pin-participant',
     resizeLargeVideo: 'resize-large-video',
     sendEndpointTextMessage: 'send-endpoint-text-message',
     sendTones: 'send-tones',
         });
     }
 
+    /**
+     * Pins a participant's video on to the stage view.
+     *
+     * @param {string} participantId - Participant id (JID) of the participant
+     * that needs to be pinned on the stage view.
+     * @returns {void}
+     */
+    pinParticipant(participantId) {
+        this.executeCommand('pinParticipant', participantId);
+    }
+
     /**
      * Removes event listener.
      *