Add Google Artifact Registry as docker registry

[Yuan325]

Is your feature request related to a problem? Please describe.
Our server is deployed in GAR. As of now, it seems like MCP registry only supports the official Docker registry.

Describe the solution you'd like
Supporting GAR.

      "registry_type": "oci",
      "registry_base_url": "https://artifactregistry.googleapis.com",
      "identifier": "<region>-docker.pkg.dev/<repo>/...",
      "version": "1.0.0",
      "file_sha256": "filesha256",
      "tag": "latest", // it will be great to support tag as well.

Describe alternatives you've considered
N/A

Additional context
N/A

[domdomegg]

Sounds good! Same probably applies as for GHCR here: #393