join method
Implementation
Future<TUIActionCallback> join(String callId) {
Completer<TUIActionCallback> completer = Completer();
String userData = Tools.generateUserData("Call_join");
_actionCallback2Future(userData, completer);
final params = [callId];
_callEngineSDK.Dart_CallEngineCallAPI(
'join'.toNativeUtf8().cast(),
jsonEncode(params).toNativeUtf8().cast(),
userData.toNativeUtf8().cast());
return completer.future;
}