chore(controlplane): s/oci_repository/cas_backend/g

[migmartri]

As part of #223, we want to enable the controlplane to be able to store other types of CAS_Backend.

• adds migrations to rename current oci_repository DB table to cas_backends.
  • it includes now a provider property and a default property
  • This default property is now used to load the default cas backend for the org
  • If the instance already has data in the table, it will keep it and mark is as OCI
• Updates the data and biz layer to meet that new DB schema
• Deprecates the oci_repositories API endpoint. It will get replaced by a cas_backends one.

All of that is backwards compatible.

Note for reviewers. This is a big patch but note that most of the code is generated.

instead ptal at

• migrations/migrate
• biz/cas_backend.go

[migmartri]

I needed to change this because of https://buf.build/docs/migration-guides/migrate-remote-generation-alpha/

[danlishka]

LGTM