Add session registration history.

Setting config `broker.session_history_retain` allows EMQX to keep track of expired sessions for the retained period.

API `GET /api/v5/sessions_count?since=1705682238` can be called to count the cluster-wide sessions which were alive (unexpired) since the provided timestamp (UNIX epoch at seconds precision).

A new gauge `cluster_sessions` is added to the metrics collection. Exposed to prometheus as

```
# TYPE emqx_cluster_sessions_count gauge
emqx_cluster_sessions_count 1234
```

NOTE: The counter can only be used for an approximate estimation as the collection and calculations are async.