Be more precise about allowed media-types for ocm oci-manifest

[zkdev]

What would you like to be added:
Precisely list allowed media-types for ocm oci-manifest.

Why is this needed:
Currently, oci-backend-spec just states that:

The component version is represented as OCI image manifest.

I assume this maps to oci image manifest spec, but I was not too sure whether image indices (or "fat manifests" / "multi-arch manifests") are allowed as well. Therefore I think, explicitly listing the technical values of allowed media-types improves spec quality, as users do not have to research on media-types and implications but rather can straight-up implement against the spec.
Also, considering media-types are versioned, an explicit list would improve on resilience as there could be an oci image manifest spec update (e.g. introduction of new media-type), which would require adjustments in ocm-spec.