[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.

该 issue 未指出任何文件、测试或入口点；请先审查 Firebase Admin Java SDK 现有的通知和身份验证接口，并明确所需的 MQTT 服务器边界。完成的要求是：就 MQTT URL 处理、通知路由和授权令牌支持确定一个经协商且可实现的范围，并识别受影响的测试。

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