RFE: bootc upgrade --check json output

[beav]

Thanks for the quick response for #245. Can the upgrade check optionally send its output as json? This makes parsing a lot easier.

Something like this would work:

{ "change_available": "True",
  "registry_path": "ostree-unverified-registry:10.0.0.1:5000/bootc-insights:latest",
  "staged_version": "abc1234",
  "available_version": "def5678" }

However, if you have other ideas for the fields, those work too.

IMO it makes sense to keep this out of the status json since it makes a network call, which may be unexpected behavior for status.

[cgwalters]

PR in #248