Implement the Referrers interface on Artifact

Signed-off-by: Bracken Dawson <abdawson@gmail.com>

manifest/artifact/ocimanifest.go

@@ -115,3 +115,15 @@ func (m *DeserializedManifest) Payload() (string, []byte, error) {
 func (m *DeserializedManifest) References() []distribution.Descriptor {
 	return m.Blobs
 }
+
+// Subject returns a pointer to the subject of this manifest or nil if there is
+// none
+func (m *DeserializedManifest) Subject() *distribution.Descriptor {
+	return m.Manifest.Subject
+}
+
+// Type returns the artifactType of this manifest if there is one, otherwise it
+// returns empty string.
+func (m *DeserializedManifest) Type() string {
+	return m.Manifest.ArtifactType
+}

manifests.go

@@ -32,6 +32,10 @@ type Referrer interface {
 	// this manifest refers to or nil if this manifest does not refer to a
 	// subject.
 	Subject() *Descriptor
+
+	// Type returns the type of the referrer if there is one, otherwise it
+	// returns empty string
+	Type() string
 }
 
 // ManifestBuilder creates a manifest allowing one to include dependencies.