Verify the Integrity and Authenticity of OCI Artifacts

Verify the integrity of the Helm Charts for OCIRepository flux resources: https://fluxcd.io/blog/2022/11/verify-the-integrity-of-the-helm-charts-stored-as-oci-artifacts-before-reconciling-them-with-flux/

more generally, verify the integrity of any OCI artifacts: https://fluxcd.io/blog/2022/10/prove-the-authenticity-of-oci-artifacts/