[inventory] Add zpool health to inventory

[karencfv]

Does exactly what it says on the tin.

Output from the API on simulated omicron:

$ curl -H "api-version: 24.0.0"  http://[::1]:58414/inventory | jq .zpools
<..>
[
  {
    "id": "84c820d7-8af9-4c79-89cf-ae07c70b0c28",
    "total_size": 17179869184,
    "health": "online"
  },
  {
    "id": "6deda61c-2bfe-452c-a018-a35ab566ae07",
    "total_size": 17179869184,
    "health": "online"
  },
  {
    "id": "46c5185c-6b09-4e8f-8da2-a65cd24b443d",
    "total_size": 17179869184,
    "health": "online"
  }
]

On an a4x2 machine I forced a fault on a zpool and this is a snippet from the inventory collection

root@oxz_switch:~# omdb db inventory collections show latest
<...>
    zpools
      1625cac3-72a8-496a-ba89-799f6f6cd274: total size: 19968 MiB health: online
      26012db4-bba8-47ed-9e57-cb4b96562c19: total size: 19968 MiB health: online
      2dd8b022-c560-42f8-acfb-154f0e8062aa: total size: 19968 MiB health: online
      619af41c-70ae-401a-9bd1-190b17cf15c8: total size: 19968 MiB health: online
      ff983224-ef9a-47e5-9d3e-57e64909a6eb: total size: 19968 MiB health: unavailable

Supersedes: #9615
Closes: #9719