New bridge notification

[JaylinYu]

when a client connect to nanomq broker after bridge has already connected successfully. when a client connect to nanomq broker before bridge connect successfully.

[adarsh-jha-dev]

Hey @JaylinYu , I'd like to work on this issue, please assign this to me

[JaylinYu]

Sure! Let me make you understand more about this issue in case you get it wrong. user wants a way to know the state of bridging. In the previous version, we rely on the online/offline notification. However, things are getting messy when nanomq restarts. Client might connect after bridging is connected. Thus, we need to cache create a new notification way for users to check the state for bridging. Either cache the online/offline msg of bridging client, or a new HTTP REST API