Add JSON output to update --check and include the version of the target container image for the Ostree Native Container path

[travier]

To be able to more easily use the output of rpm-ostree update --check in Plasma Discover, add JSON output to update --check and include the version of the target container image for the Ostree Native Container path, similarly to how it is displayed in the classic ostree path.

Host system details

Latest rpm-ostree.

Expected behavior

# rpm-ostree update --check
...
Available update: Version: .....
Layers:
...

# rpm-ostree update --check --json
{
    "origin": "ostree-unverified-image:registry:quay.io/foo/bar/latest"
    "version": "40.X.Y",
    "layers": {
        "total": {
            "count": "",
            "size": "" 
        },
        "removed": {
            "count": "",
            "size": "" 
        },
        "added": {
            "count": "",
            "size": "" 
        },
    }
}

# or when no new version is found:
# rpm-ostree update --check --json
{}

Steps to reproduce it

Check for updates on a system using Ostree Native Container.

Would you like to work on the issue?

Yes, will start working on this.