Remove OCI backend dependency from Controlplane

[migmartri]

Currently, the controlplane has a dependency on a OCI backend provider so it can upload/download the generated attestation as well as downloading SBOMs for re-routing to Dependency-Track.

We have a component that it's already handling OCI upload/downloads which is the Artifact CAS, and this is leveraged by the CLI already.

We should delegate OCI access from the control plane to the artifact CAS and leverage its API instead this means.

• Extract the CAS client code that it's currently in the CLI and move it to a top level package chore: decouple cas-client file upload #43
• Implement CAS upload use-case for attestation feat(controlplane): support for sending attestation through CAS #45
• Implement CAS download feat(controlplane): download attestation using CAS #46
• Download attestation using CAS feat(controlplane): download attestation using CAS #46
• Download SBOMs using CAS
• Remove credentials.Reader and backend provider use from the control plane