protocol MessagingDelegate : NSObjectProtocol
A protocol to handle token update or data message delivery from FCM.
This method will be called once a token is available, or has been refreshed. Typically it will be called once per app start, but may be called more often, if token is invalidated or updated. In this method, you should perform operations such as:
Uploading the FCM token to your application server, so targeted notifications can be sent.
Subscribing to any topics.
optional func messaging(_ messaging: Messaging, didReceiveRegistrationToken fcmToken: String)