how to get mqtt client status

1、当前问题
当前/session ，get 请求 的 api 接口是获取单个clent 的状态，如果设备量多，会导致请求次数过多

2、 期望结果
client 的状态 回调接口或者 topic 的通知接口
希望实现 client 上线和离线状态可以通过回调接口 告诉 应用程序，或者 应用程序通过订阅某个 topic 来获取

Start by reviewing the existing /session GET API for single-client status, then compare the requested callback and topic-notification approaches. Done requires an agreed mechanism for applications to receive client online and offline status without querying each client individually.

Written by the indexing model from the issue text.