API to retrive tag's digests

Add an interface alongside TagStore that provides API to retreive
digests of all manifests that a tag historically pointed to. It also
includes currently linked tag.

Signed-off-by: Manish Tomar <manish.tomar@docker.com>

tags.go

@@ -2,6 +2,8 @@ package distribution
 
 import (
 	"context"
+
+	digest "github.com/opencontainers/go-digest"
 )
 
 // TagService provides access to information about tagged objects.
@@ -25,3 +27,10 @@ type TagService interface {
 	// Lookup returns the set of tags referencing the given digest.
 	Lookup(ctx context.Context, digest Descriptor) ([]string, error)
 }
+
+// TagIndexes proves method to retreive all the digests that a tag historically pointed to
+type TagIndexes interface {
+	// Indexes returns set of digests that this tag historically pointed to. This also includes
+	// currently linked digest. There is no ordering guaranteed
+	Indexes(ctx context.Context, tag string) ([]digest.Digest, error)
+}