Fix TypeScript optional parameter on RtcEngine's options

lib/types.d.ts

     appid: String;
     channelProfile: number;
     videoEncoderConfig: VideoEncoderConfig;
-    dualStream: boolean;
-    mode: number;
+    dualStream?: boolean;
+    mode?: number;
     clientRole: number;
     audioProfile: number;
     audioScenario: number;

src/types.ts

   appid: String,
   channelProfile: number,
   videoEncoderConfig: VideoEncoderConfig,
-  dualStream: boolean,
-  mode: number,
+  dualStream?: boolean,
+  mode?: number,
   clientRole: number,
   audioProfile: number,
   audioScenario: number,