Add zpool health monitoring support

Zpool monitoring is completely missing from zabbix, and many of the people are  running zfs. Zpool health is clearly displayed by zpool-status(8). There should be discovery items and a default template which creates the items, triggers on non-online pools/vdevs/etc.

Health status is one of the most crucial part of monitoring solutions, and unfortunately zabbix doesn't offer a lot of help out of the box here.

So, could you please add zpool monitoring? It's the same across all systems since everyone has moved to openzfs.