Override appengine_v1::ServicesClient 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::ServicesClient(
 google::cloud::appengine_v1::MakeServicesConnection(options));