new(artifact/manifest): add manifest command

[alacuku]

It allows to fetch the manifest for a given artifact based on it's platform.

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

/kind flaky-test

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area library

/area cli

/area tests

/area examples

What this PR does / why we need it:
Add a new command for artifacts: falcoctl artifact manifest which fetches the manifest layer for a given artifact.

❯ falcoctl artifact manifest k8saudit --platform linux/arm64 | jq
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "config": {
    "mediaType": "application/vnd.cncf.falco.plugin.config.v1+json",
    "digest": "sha256:75ff683c3b711f2425e41faeb11830eae49b3ad9e76087cf4c49f2dad0c5e71f",
    "size": 102
  },
  "layers": [
    {
      "mediaType": "application/vnd.cncf.falco.plugin.layer.v1+tar.gz",
      "digest": "sha256:8987a995fa09518aebc488ba549448166d605596c2d6478c10415a9d9f5f05dd",
      "size": 3837491,
      "annotations": {
        "org.opencontainers.image.title": "k8saudit-0.6.1-linux-aarch64.tar.gz"
      }
    }
  ],
  "annotations": {
    "org.opencontainers.image.created": "2023-09-21T07:47:28Z",
    "org.opencontainers.image.source": "https://github.com/falcosecurity/plugins.git"
  }
}

Which issue(s) this PR fixes:
#333

Fixes #
#333
Special notes for your reviewer:

[FedeDP]

/approve

[poiana]

LGTM label has been added.

Git tree hash: 945f3dc9e58b5ff717b7f775c3176f7755e1353b

[FedeDP]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alacuku, FedeDP

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[poiana]

LGTM label has been added.

Git tree hash: 14142df01d2882d8c154571fa5486ba08e147792