Override InstanceAdminClient Default Endpoint

  // 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::spanner_admin::InstanceAdminClient(
      google::cloud::spanner_admin::MakeInstanceAdminConnection(options));