oci: Introduce Flux media types

[stefanprodan]

This PR changes the media type of the OCI artifacts produced by Flux to:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "config": {
    "mediaType": "application/vnd.cncf.flux.config.v1+json",
    "size": 233,
    "digest": "sha256:1b80ecb1c04d4a9718a6094a00ed17b76ea8ff2bb846695fa38e7492d34f505c"
  },
  "layers": [
    {
      "mediaType": "application/vnd.cncf.flux.content.v1.tar+gzip",
      "size": 19081,
      "digest": "sha256:46c2b334705cd08db1a6fa46f860cd3364fc1a3636eea37a9b35537549086a1c"
    }
  ],
  "annotations": {
    "org.opencontainers.image.created": "2023-02-10T09:06:09Z",
    "org.opencontainers.image.revision": "test",
    "org.opencontainers.image.source": "test"
  }
}

Ref: fluxcd/flux2#3575

[hiddeco]

Was looking forward to this since the initial introduction of the feature. Looks good to me, thanks Stefan 🙇

[darkowlzz]

LGTM!
Left a comment about a minor improvement in test.
Thanks for silencing the verbose registry logs.