[Feat]: `agent-connection-dropped` event when EMQX reports disconnection

[ralphm]

Problem

No response

Description

We recently made a change that made Cloud better aware that an ACLK connection was dropped. However, as there are two sources of information, it could be that the signal from EMQX comes in later than the in-band signal from the bridges, causing us to lose the reason EMQX thinks the connection was dropped.

Importance

really want

Value proposition

1. Better insight in why connections were dropped.

Proposed implementation

Introduce a new event agent-connection-dropped that is emitted solely from the EMQX system topic, with the given reason.