[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.

El issue no menciona archivos, pruebas ni puntos de entrada; comienza revisando las superficies existentes de notificaciones y autenticación del Firebase Admin Java SDK y aclarando el límite deseado del servidor MQTT. Se considera terminado cuando exista un alcance acordado e implementable para el manejo de URL de MQTT, el enrutamiento de notificaciones y la compatibilidad con tokens de autorización, y se hayan identificado las pruebas afectadas.

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