New command: artifact manifest

[alacuku]

What would you like to be added:
Add a new command for artifacts that fetches and prints to stdout the manifest layer:

falcoctl artifact manifest ghcr.io/falcosecurity/plugins/ruleset/k8saudit:latest

Output:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "config": {
    "mediaType": "application/vnd.cncf.falco.rulesfile.config.v1+json",
    "digest": "sha256:af1b99ae2aa1d1ae2d452bc66c36ce2d30a939f61111c82cd227456b5318728a",
    "size": 249
  },
  "layers": [
    {
      "mediaType": "application/vnd.cncf.falco.rulesfile.layer.v1+tar.gz",
      "digest": "sha256:36321b3f1d7969926073a4d40bbbb7b4b28805b038c067f140795210ab641161",
      "size": 6471,
      "annotations": {
        "org.opencontainers.image.title": "k8saudit-rules-0.6.1.tar.gz"
      }
    }
  ],
  "annotations": {
    "org.opencontainers.image.created": "2023-09-21T07:47:36Z",
    "org.opencontainers.image.source": "https://github.com/falcosecurity/plugins.git"
  }
}

[alacuku]

/assign @alacuku