OCI media types; annotation support; oci index

Signed-off-by: Mike Brown <brownwm@us.ibm.com>

blobs.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/docker/distribution/reference"
 	"github.com/opencontainers/go-digest"
+	"github.com/opencontainers/image-spec/specs-go/v1"
 )
 
 var (
@@ -72,6 +73,13 @@ type Descriptor struct {
 	// URLs contains the source URLs of this content.
 	URLs []string `json:"urls,omitempty"`
 
+	// Annotations contains arbitrary metadata relating to the targeted content.
+	Annotations map[string]string `json:"annotations,omitempty"`
+
+	// Platform describes the platform which the image in the manifest runs on.
+	// This should only be used when referring to a manifest.
+	Platform *v1.Platform `json:"platform,omitempty"`
+
 	// NOTE: Before adding a field here, please ensure that all
 	// other options have been exhausted. Much of the type relationships
 	// depend on the simplicity of this type.