Add scanning support for OCI images

Issue

• Images of manifestType application/vnd.oci.image.index.v1+json could not be scanned.
• The error message indicated problems retrieving metadata and manifest digests, resulting in an HTTP 404 response

failed to scan image: image enrichment error: error getting metadata for image: ghcr.io/bitnami-labs/sealed-secrets-controller:latest error: getting metadata from registry: "Github": Failed to get the manifest digest : Head "https://ghcr.io/v2/bitnami-labs/sealed-secrets-controller/manifests/latest": http: non-successful response (status=404 body="")