public Optional<BlobDescriptor> checkBlob(DescriptorDigest blobDigest)
      throws IOException, RegistryException {
    BlobChecker blobChecker = new BlobChecker(registryEndpointRequestProperties, blobDigest);
    return callRegistryEndpoint(blobChecker);
  }
Check if a blob is on the registry.

@param blobDigest the blob digest to check for
@return the BLOB's {@link BlobDescriptor} if the BLOB exists on the registry, or {@link
    Optional#empty()} if it doesn't
@throws IOException if communicating with the endpoint fails
@throws RegistryException if communicating with the endpoint fails
jib-core/src/main/java/com/google/cloud/tools/jib/registry/RegistryClient.java