Fix pg_autoctl show settings --json output.

[DimCitus]

The formation context parts should also be an array, even when we know it
contains a single element for now.

Fixes #694

[DimCitus]

With this fix we have:

$ pg_autoctl show settings --json | jq '.formation'
[
  {
    "value": "1",
    "context": "formation",
    "node_id": null,
    "setting": "number_sync_standbys",
    "group_id": null,
    "nodename": "default"
  }
]

And that makes sense, and it's the same format as the other parameters, using JSON objects in an EAV approach.

[JelteF]

Makes sense. Needs to be called out as a breaking change in the changelog though.