fix(validation): do not enforce entry mediatypes

[whatsacomputertho]

In this PR, I soften the check on manifest list entry mediaTypes. Previously in the v2s2 manifest list and OCI index validation logic I checked the list entries for whether their mediaTypes align with the parent list mediaType. I.e. if it's a v2s2 manifest list then it better not include OCI manifests, or vice versa. But it does not seem that the official containerization libraries make this assertation so I am comfortable softening this check.

This eliminates the final blocker for @matejvasek @cmoulliard to use this library to inspect the properties of their base image from ghcr.io.

For reference, see:

• Manifest lists with manifests of different mediaType should be valid #32
• Fails to pull a public image from: ghcr.io/matejvasek/builder-ubi8-base:latest #30

[whatsacomputertho]

Looks like unit tests expected that condition, may need to clean up some old asserts