widget: add ZFS pool status API and dashboard widget

[TuEye]

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Is your feature request related to a problem? Please describe.

Currently the dashboard has no widget to show the status of ZFS pools. Additionally, there is also no API endpoint to fetch this information. For monitoring purposes it would be useful to include both: the widget as an easy way to see if a ZFS pool is healthy, and the API endpoint for this widget and external monitoring.

Describe the solution you like

Add a simple ZFS status with "zpool status -j --json-int" and expose it through the diagnostics API, similar to other system information such as "system_disk". Then the ZFS widget can use this endpoint to show the following information:

• pool health/state
• number of errors in the pool
• the date of the last scrub or resilver

Describe alternatives you considered

External ZFS monitoring with Zabbix or node_exporter is already possible. But until now there is no way to show this information on the dashboard.

Additional context

I've already been working on an implementation and will be opening a pull request shortly.

[sopex]

Very nice idea!