feat(external_ap) add api call to get live stream url

modules/API/API.js

             });
             break;
         }
+        case 'get-livestream-url': {
+            const state = APP.store.getState();
+            const conference = getCurrentConference(state);
+            let livestreamUrl;
+
+            if (conference) {
+                const activeSession = getActiveSession(state, JitsiRecordingConstants.mode.STREAM);
+
+                livestreamUrl = activeSession?.liveStreamViewURL;
+            } else {
+                logger.error('Conference is not defined');
+            }
+            callback({
+                livestreamUrl
+            });
+            break;
+        }
         default:
             return false;
         }

modules/API/external/external_api.js

         return getCurrentDevices(this._transport);
     }
 
+    /**
+     * Returns the current livestream url.
+     *
+     * @returns {Promise} - Resolves with the current livestream URL if exists, with
+     * undefined if not and rejects on failure.
+     */
+    getLivestreamUrl() {
+        return this._transport.sendRequest({
+            name: 'get-livestream-url'
+        });
+    }
+
     /**
      * Returns the conference participants information.
      *