Override compute_forwarding_rules_v1::ForwardingRulesClient Retry Policies

  auto options = google::cloud::Options{}
                     .set<google::cloud::compute_forwarding_rules_v1::
                              ForwardingRulesConnectionIdempotencyPolicyOption>(
                         CustomIdempotencyPolicy().clone())
                     .set<google::cloud::compute_forwarding_rules_v1::
                              ForwardingRulesRetryPolicyOption>(
                         google::cloud::compute_forwarding_rules_v1::
                             ForwardingRulesLimitedErrorCountRetryPolicy(3)
                                 .clone())
                     .set<google::cloud::compute_forwarding_rules_v1::
                              ForwardingRulesBackoffPolicyOption>(
                         google::cloud::ExponentialBackoffPolicy(
                             /*initial_delay=*/std::chrono::milliseconds(200),
                             /*maximum_delay=*/std::chrono::seconds(45),
                             /*scaling=*/2.0)
                             .clone());
  auto connection = google::cloud::compute_forwarding_rules_v1::
      MakeForwardingRulesConnectionRest(options);

  // c1 and c2 share the same retry policies
  auto c1 = google::cloud::compute_forwarding_rules_v1::ForwardingRulesClient(
      connection);
  auto c2 = google::cloud::compute_forwarding_rules_v1::ForwardingRulesClient(
      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_forwarding_rules_v1::ForwardingRulesClient(
      connection,
      google::cloud::Options{}
          .set<google::cloud::compute_forwarding_rules_v1::
                   ForwardingRulesRetryPolicyOption>(
              google::cloud::compute_forwarding_rules_v1::
                  ForwardingRulesLimitedTimeRetryPolicy(std::chrono::minutes(5))
                      .clone()));

  // You can also override the policies in a single call:
  // c3.SomeRpc(..., google::cloud::Options{}
  //     .set<google::cloud::compute_forwarding_rules_v1::ForwardingRulesRetryPolicyOption>(
  //       google::cloud::compute_forwarding_rules_v1::ForwardingRulesLimitedErrorCountRetryPolicy(10).clone()));

class CustomIdempotencyPolicy
    : public google::cloud::compute_forwarding_rules_v1::
          ForwardingRulesConnectionIdempotencyPolicy {
 public:
  ~CustomIdempotencyPolicy() override = default;
  std::unique_ptr<google::cloud::compute_forwarding_rules_v1::
                      ForwardingRulesConnectionIdempotencyPolicy>
  clone() const override {
    return std::make_unique<CustomIdempotencyPolicy>(*this);
  }
  // Override inherited functions to define as needed.
};