Allow conditional fetching of manifests with the registry client.

Add a functional argument to pass a digest to (ManifestService).GetByTag().
If the digest matches an empty manifest and nil error are returned.

See 1bc740b0d5 for server implementation.

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

registry.go

@@ -37,6 +37,9 @@ type Namespace interface {
 	Repository(ctx context.Context, name string) (Repository, error)
 }
 
+// ManifestServiceOption is a function argument for Manifest Service methods
+type ManifestServiceOption func(ManifestService) error
+
 // Repository is a named collection of manifests and layers.
 type Repository interface {
 	// Name returns the name of the repository.
@@ -84,7 +87,7 @@ type ManifestService interface {
 	ExistsByTag(tag string) (bool, error)
 
 	// GetByTag retrieves the named manifest, if it exists.
-	GetByTag(tag string) (*manifest.SignedManifest, error)
+	GetByTag(tag string, options ...ManifestServiceOption) (*manifest.SignedManifest, error)
 
 	// TODO(stevvooe): There are several changes that need to be done to this
 	// interface: