EnsureReferences will enrich the VerifyOpts with the image digest and signature reference.

SimpleDigest is an adaption of crane.Digest - it returns the sha256 hash of the remote image at ref.

UnmarshalPEMToPublicKey converts a PEM-encoded byte slice into a crypto.PublicKey.

VerifySignature checks if the image is signed with the given key and if the annotations match the given rules This does a lot of image and image manifest juggling to fetch artifacts, digests, etc.

If an image was signed by `acorn image sign foo/bar:v1`, this annotation should be set to `foo/bar:v1` (the payload usually only includes the image digest).