Support Multi-Variant OCI Images

[hardikdr]

Summary

Add support for multi-variant OCI images, allowing a single image reference to encapsulate different variants (e.g., baremetal and VM), similar to how multi-architecture images work today.

Basic Example (for illustration only)

{
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "...",
      "platform": {
        "architecture": "amd64",
        "os": "linux",
        "variant": "baremetal"
      }
    },
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "...",
      "platform": {
        "architecture": "amd64",
        "os": "linux",
        "variant": "vm"
      }
    }
  ]
}

Motivation

Enable distribution of a single image reference that supports multiple boot environments (baremetal, VM, etc.)