API breaking change: Spin off messaging API into its own service

I broke out the module's API into a separate class, FirebasePhpMessagingApi. This way, if you like to call a single function that does everything but has a ton of arguments, you can use the API, and if you would rather keep things simple and use kreait/firebase-php directly, the API isn't there and won't get in your way.

Automatically closed - issue fixed for 2 weeks with no activity.