This change ensures that if a manifest list is requested with an accepts header for a *schema 2* manifest, the legacy manifest (if any) is returned as schema 2 if it was pushed as a schema 2 manifest (rather than being auto-converted to schema 1)
This adds additional required properties and methods to the Docker schema interface to allow us to treat both schema1 and schema2 manifests and lists logically equivalent from the OCI mode perspective
TagManifests can (apparently, in very rare scenarios) share manifests with the exact same digests, so we need to support that case in the backfill worker. We also need to remove a unique constraint on the manifest column in the mapping table to support this case.
After discussion, we decided the best solution for the missing content checksum problem was to lookup the proper blobs in the repository and, if not present, mark the manifest as broken, as this would reflect the actual issue the user faces if they pull the repository tag today via V2
