[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.

該 issue 未指出任何檔案、測試或進入點；請先檢視 Firebase Admin Java SDK 現有的通知與驗證介面，並釐清所需的 MQTT 伺服器邊界。完成的要求是：針對 MQTT URL 處理、通知路由及授權權杖支援，確定一個經協議且可實作的範圍，並識別受影響的測試。

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