artifacts: initial support for artifacts/notaryv2 spec

https://github.com/oras-project/artifacts-spec
https://github.com/notaryproject/notaryproject

Fixes issue #264

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

pkg/storage/storage.go

@@ -3,6 +3,7 @@ package storage
 import (
 	"io"
 
+	"github.com/notaryproject/notation-go-lib"
 	"github.com/opencontainers/go-digest"
 )
 
@@ -40,4 +41,5 @@ type ImageStore interface {
 	DeleteBlob(repo string, digest string) error
 	GetIndexContent(repo string) ([]byte, error)
 	GetBlobContent(repo, digest string) ([]byte, error)
+	GetReferrers(repo, digest string, mediaType string) ([]notation.Descriptor, error)
 }