[Question] MQTT plugin implementation discussion

### Question

I have a question regarding the incomplete MQTT plugin in the current ShenYu gateway: does the plugin forward messages from MQTT Clients to an external MQTT Broker, or does it implement an internal MQTT Broker within the gateway to communicate directly with MQTT Clients?

I would greatly appreciate it if contributors could help address my questions. Thank you so much!

No files, tests, or entry points are named. First clarify whether the incomplete MQTT plugin should proxy clients to an external broker or provide an internal broker; the issue is done when that architecture question is answered and a concrete implementation scope is identified.

Written by the indexing model from the issue text.