Proposed fix for image content store

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

oci/client.go

@@ -11,11 +11,12 @@ import (
 // Client interface used by SpecOpt
 type Client interface {
 	SnapshotService(snapshotterName string) snapshot.Snapshotter
-	ContentStore() content.Store
 }
 
 // Image interface used by some SpecOpt to query image configuration
 type Image interface {
 	// Config descriptor for the image.
 	Config(ctx context.Context) (ocispec.Descriptor, error)
+	// ContentStore provides a content store which contains image blob data
+	ContentStore() content.Store
 }