Override @c Publisher Default Endpoint

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::Options;
  [](std::string project_id, std::string topic_id) {
    auto topic = pubsub::Topic(std::move(project_id), std::move(topic_id));
    return pubsub::Publisher(pubsub::MakePublisherConnection(
        std::move(topic), Options{}.set<google::cloud::EndpointOption>(
                              "private.googleapis.com")));
  }