Consider using OCI compliant media-types

[ChristianCiach]

Describe the problem/challenge you have

I am trying to push Imgpkg artifacts to a Zot registry. Zot registry describes itself as a "vendor neutral" OCI registry. In practice this just means that Zot refuses to accept any image or artifact that uses a Docker (or any other non-OCI) specific mediaType.

For more information about this Zot's refusal to work with Docker artifacts, see this issue:

• FR: loose mode allowing Docker mediaTypes  project-zot/zot#724

You can already see where I am going with this:

$ imgpkg push -i localhost:5000/test-bundle:v1.0.0 -f anyfile.txt --registry-insecure

imgpkg: Error: Writing 'localhost:5000/test-bundle:v1.0.0':
  Writing image:
    PUT http://localhost:5000/v2/test-bundle/manifests/v1.0.0:
      MANIFEST_INVALID: manifest invalid; map[description:During upload, manifests undergo several checks ensuring validity. If those checks fail, this error MAY be returned, unless a more specific error is included. The detail will contain information the failed validation. mediaType:application/vnd.docker.distribution.manifest.v2+json]

Describe the solution you'd like

Since it's probably unlikely for Zot to ever accept mediaTypes that are not defined by OCI, I would like imgpkg to (optionally) support the creation of bundles using standard OCI mediaTypes for its manifests and layers.

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[rchincha]

For more information about this Zot's refusal to work with Docker artifacts, see this issue:

Regardless of what the zot project does, I respectfully urge the carvel team to look at becoming OCI compliant.
For reference, helm already is: https://helm.sh/docs/topics/registries/

[praveenrewar]

@ChristianCiach I believe you would be interested in this proposal.