Override compute_region_instance_groups_v1::RegionInstanceGroupsClient 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::compute_region_instance_groups_v1::
      RegionInstanceGroupsClient(
          google::cloud::ExperimentalTag{},
          google::cloud::compute_region_instance_groups_v1::
              MakeRegionInstanceGroupsConnectionRest(
                  google::cloud::ExperimentalTag{}, options));