Allow OCI manifest database only validation
Related to: #240 (closed)
This MR allows database-backed validation of OCI manifests.
Additionally, some validation logic between schema2 and OCI manifests has been moved into a single function. There are quite a few similarities between the validation of these two types of manifest, but due to the different respective mediatypes and concrete types, if does not appear to be prudent to aggressively merge this logic.
Edited by Hayley Swimelar