Override compute_target_vpn_gateways_v1::TargetVpnGatewaysClient Authentication Defaults

  [](std::string const& keyfile) {
    auto is = std::ifstream(keyfile);
    is.exceptions(std::ios::badbit);  // Minimal error handling in examples
    auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
    auto options =
        google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>(
            google::cloud::MakeServiceAccountCredentials(contents));
    return google::cloud::compute_target_vpn_gateways_v1::
        TargetVpnGatewaysClient(
            google::cloud::ExperimentalTag{},
            google::cloud::compute_target_vpn_gateways_v1::
                MakeTargetVpnGatewaysConnectionRest(
                    google::cloud::ExperimentalTag{}, options));
  }