Embed "Unimplemented" structs as proto recommended

Embedding these structs will be mandatory after migratring off from
gogo/protobuf (see #6564).

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

services/sandbox/controller_service.go

@@ -59,6 +59,7 @@ func init() {
 
 type controllerService struct {
 	local api.ControllerClient
+	api.UnimplementedControllerServer
 }
 
 var _ api.ControllerServer = (*controllerService)(nil)

services/sandbox/store_service.go

@@ -55,6 +55,7 @@ func init() {
 
 type sandboxService struct {
 	local api.StoreClient
+	api.UnimplementedStoreServer
 }
 
 var _ api.StoreServer = (*sandboxService)(nil)