Support externally-stored artifact content

[jsenko]

There are use-cases where Registry does not have to or can't store artifact content in the database, instead using an external storage system. Use-cases:

• Model Registry - content is stored in S3 buckets
• OCI artifacts support - content is stored e.g. in quay.io
• GitOps - content can be read from a repository on the filesystem

Registry will keep storing artifact metadata in the database, for example to support search. Considerations:

• Should we still support getting the content via Registry REST API (forwarding?) or provide a user friendly information on how to get the data separately. This also depends on whether the Registry will need to access the data, e.g. to extract metadata or run rules, but on big files this may be too expensive.