Override @c Subscriber Default Authentication

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::Options;
  [](std::string project_id, std::string subscription_id,
     std::string const& keyfile) {
    auto is = std::ifstream(keyfile);
    is.exceptions(std::ios::badbit);
    auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
    auto subscription =
        pubsub::Subscription(std::move(project_id), std::move(subscription_id));
    return pubsub::Subscriber(pubsub::MakeSubscriberConnection(
        std::move(subscription),
        Options{}.set<google::cloud::UnifiedCredentialsOption>(
            google::cloud::MakeServiceAccountCredentials(contents))));
  }