Support for OCI backend in addition to git

[ciaranjohnston]

The "OCI" backend type is a supported backend in the Porch API, however the implementation is incomplete and / or untested. This should either be:

• fixed and documented as a "first-class" backend type
• documented as "broken"
• removed as an option

This issue is a result of a discussion around this user story proposal:
https://docs.google.com/document/d/1d4USe4nOcOQAb0ULCMGSPtC72f0dbV0SguPCPEGYaac/edit?tab=t.0

[ciaranjohnston]

Potentially only applicable to blueprints - once you do mutations and variants you are rewriting back to the repository with new names. Preferable to keep and document as "not functional" for now to ensure the support for multiple repo types can be maintained.