[FR] MQTT integration

I'm trying to make some synchronization between my mqtt server and the firebase notification, when to notify the client through firebase and when to send through mqtt, which does mean that i can't really use the topic notification. It would be excellent, if i pass the mqtt url to firebase and can handle that stuff (even with external not in-house mqtt servers). I'm customizing an mqtt server with additional plugins to do the switch on/off notification dynamically and authorizing the wss client also with the firebase athorization token. (inially i have done it because no firebase admin sdk is avalable in C/C++, and it would have been hard to port onto C/C++), but the same with erlang. For out-of-box simple mobile chat it would quite usefull, if firebase can cooperate with such a server.

The issue names no files, tests, or entry points; begin by reviewing the Firebase Admin Java SDK's existing notification and authentication surfaces and clarifying the desired MQTT-server boundary. Done requires an agreed, implementable scope for MQTT URL handling, notification routing, and authorization-token support, with affected tests identified.

Written by the indexing model from the issue text.