Override @c SubscriptionAdminClient Default Authentication

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