support mod_aws_tts with engine parameter

lib/synth-audio.js

@@ -333,6 +333,7 @@ const synthPolly = async(createHash, retrieveHash, logger,
       }
     }
     if (region) params += `,region=${region}`;
+    if (engine) params += `,engine=${engine}`;
     params += '}';
 
     return {