Override compute_network_attachments_v1::NetworkAttachmentsClient Retry Policies

  auto options =
      google::cloud::Options{}
          .set<google::cloud::compute_network_attachments_v1::
                   NetworkAttachmentsConnectionIdempotencyPolicyOption>(
              CustomIdempotencyPolicy().clone())
          .set<google::cloud::compute_network_attachments_v1::
                   NetworkAttachmentsRetryPolicyOption>(
              google::cloud::compute_network_attachments_v1::
                  NetworkAttachmentsLimitedErrorCountRetryPolicy(3)
                      .clone())
          .set<google::cloud::compute_network_attachments_v1::
                   NetworkAttachmentsBackoffPolicyOption>(
              google::cloud::ExponentialBackoffPolicy(
                  /*initial_delay=*/std::chrono::milliseconds(200),
                  /*maximum_delay=*/std::chrono::seconds(45),
                  /*scaling=*/2.0)
                  .clone());
  auto connection = google::cloud::compute_network_attachments_v1::
      MakeNetworkAttachmentsConnectionRest(google::cloud::ExperimentalTag{},
                                           options);

  // c1 and c2 share the same retry policies
  auto c1 =
      google::cloud::compute_network_attachments_v1::NetworkAttachmentsClient(
          google::cloud::ExperimentalTag{}, connection);
  auto c2 =
      google::cloud::compute_network_attachments_v1::NetworkAttachmentsClient(
          google::cloud::ExperimentalTag{}, connection);

  // You can override any of the policies in a new client. This new client
  // will share the policies from c1 (or c2) *except* for the retry policy.
  auto c3 =
      google::cloud::compute_network_attachments_v1::NetworkAttachmentsClient(
          google::cloud::ExperimentalTag{}, connection,
          google::cloud::Options{}
              .set<google::cloud::compute_network_attachments_v1::
                       NetworkAttachmentsRetryPolicyOption>(
                  google::cloud::compute_network_attachments_v1::
                      NetworkAttachmentsLimitedTimeRetryPolicy(
                          std::chrono::minutes(5))
                          .clone()));

  // You can also override the policies in a single call:
  // c3.SomeRpc(..., google::cloud::Options{}
  //     .set<google::cloud::compute_network_attachments_v1::NetworkAttachmentsRetryPolicyOption>(
  //       google::cloud::compute_network_attachments_v1::NetworkAttachmentsLimitedErrorCountRetryPolicy(10).clone()));

class CustomIdempotencyPolicy
    : public google::cloud::compute_network_attachments_v1::
          NetworkAttachmentsConnectionIdempotencyPolicy {
 public:
  ~CustomIdempotencyPolicy() override = default;
  std::unique_ptr<google::cloud::compute_network_attachments_v1::
                      NetworkAttachmentsConnectionIdempotencyPolicy>
  clone() const override {
    return std::make_unique<CustomIdempotencyPolicy>(*this);
  }
  // Override inherited functions to define as needed.
};