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/namespaces/service.go

@@ -54,6 +54,7 @@ func init() {
 
 type service struct {
 	local api.NamespacesClient
+	api.UnimplementedNamespacesServer
 }
 
 var _ api.NamespacesServer = &service{}