[Bug] - For the external_events_app, register the serverless event handlers in manifest

[ravirajsubramanian]

Describe the bug
From platform version 2.2, the serverless events and Server Method Invocation handlers have to be registered in the app manifest. The app is in platform version 2.2 but the events are not registered yet. Also, the events registration has to be removed from the server.js file.

Link to the bug'd Sample App
https://github.com/freshworks/marketplace-sample-apps/tree/master/Freshworks-Samples/App-Development-Features/Serverless-Features/external_events_app