add completion handler callback to addNotificationRequest

[tavour-frank]

This wires up the completionHandler for addNotificationRequest across the bridge, exposing any error to the JS side, and allowing callers to take any actions needed on success/failure for addNotificationRequest.

[tavour-frank]

As users of this library, this is particularly useful for us in wiring up business analytics to track successful + failed calls when leveraging the addNotificationRequest method