Override appengine_v1::DomainMappingsClient Endpoint Configuration

  // This configuration is common with Private Google Access:
  //     https://cloud.google.com/vpc/docs/private-google-access
  auto options = google::cloud::Options{}.set<google::cloud::EndpointOption>(
      "private.googleapis.com");
  auto client = google::cloud::appengine_v1::DomainMappingsClient(
      google::cloud::appengine_v1::MakeDomainMappingsConnection(options));