updatePushConfiguration abstract method
Update the push configuration with a new endpoint.
if endpoint is null, the subscription stops delivering messages
automatically, and becomes a pull subscription, if it isn't already.
If endpoint is not null, the subscription will be a push
subscription, if it wasn't already, and Pub/Sub will start automatically
delivering message to the endpoint URI.
Returns a Future which completes when the operation completes.
Implementation
Future updatePushConfiguration(Uri endpoint);