Override @c SubscriptionAdminClient Default Endpoint

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::Options;
  []() {
    return pubsub::SubscriptionAdminClient(
        pubsub::MakeSubscriptionAdminConnection(
            Options{}.set<google::cloud::EndpointOption>(
                "private.googleapis.com")));
  }