Add support for OCI artifact signature verification

With the introduction of sylva-units-operator, we've dropped the support for OCI artifact signature verification, we need to implement that feature in the operator. Note that previous implementation was partial since we were only verifying the kustomize-units oci artifacts, not the one used for sylva-units helmRelease. Thanks to the implementation of !371 (merged) and !450 (merged), we'll now be able to check the signature of these two artifacts.