Override @c Subscriber Default Endpoint

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::Options;
  [](std::string project_id, std::string subscription_id) {
    auto subscription =
        pubsub::Subscription(std::move(project_id), std::move(subscription_id));
    return pubsub::Subscriber(pubsub::MakeSubscriberConnection(
        std::move(subscription), Options{}.set<google::cloud::EndpointOption>(
                                     "private.googleapis.com")));
  }