feat(controlplane): update cas-backends

[migmartri]

Update CAS backend command/API endpoint.

This command is safer than the previous upsert approach in set-oci-repo. This update command only allows you to change the description, the default status and rotate its credentials. You can not change the actual location of the repository.

Some features

• if the user wants to remove the default status, it will be warned

$ chainloop cas-backend update oci --id 4e015fe8-658b-4773-9275-bd8771927fbf          
This is the default CAS backend and your are setting default=false.
Please confirm to continue y/N: 

• You can now technically have no CAS backends enabled, this today will make the controlplane complain about that requirement, but in the future this will not be a requirement at all because of Inline CAS backend #201

Refs #223

[danlishka]

LGTM