code: func (i ImageManifest) Payload() (string, []byte, error) {
  	// TODO: If available, read content from a content store by digest
  	switch {
  	case i.SchemaV2Manifest != nil:
  		return i.SchemaV2Manifest.Payload()
  	default:
  		return "", nil, errors.Errorf("%s has no payload", i.Ref)
  	}
  }
docs: // Payload returns the media type and bytes for the manifest