Override iam_v2::PoliciesClient Retry Policies

This shows how to override the retry policies for iam_v2::PoliciesClient:

 auto options =
      google::cloud::Options{}
          .set<
              google::cloud::iam_v2::PoliciesConnectionIdempotencyPolicyOption>(
              CustomIdempotencyPolicy().clone())
          .set<google::cloud::iam_v2::PoliciesRetryPolicyOption>(
              google::cloud::iam_v2::PoliciesLimitedErrorCountRetryPolicy(3)
                  .clone())
          .set<google::cloud::iam_v2::PoliciesBackoffPolicyOption>(
              google::cloud::ExponentialBackoffPolicy(
                  /*initial_delay=*/std::chrono::milliseconds(200),
                  /*maximum_delay=*/std::chrono::seconds(45),
                  /*scaling=*/2.0)
                  .clone());
  auto connection = google::cloud::iam_v2::MakePoliciesConnection(options);

  // c1 and c2 share the same retry policies
  auto c1 = google::cloud::iam_v2::PoliciesClient(connection);
  auto c2 = google::cloud::iam_v2::PoliciesClient(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::iam_v2::PoliciesClient(
      connection, google::cloud::Options{}
                      .set<google::cloud::iam_v2::PoliciesRetryPolicyOption>(
                          google::cloud::iam_v2::PoliciesLimitedTimeRetryPolicy(
                              std::chrono::minutes(5))
                              .clone()));

  // You can also override the policies in a single call:
  // c3.SomeRpc(..., google::cloud::Options{}
  //     .set<google::cloud::iam_v2::PoliciesRetryPolicyOption>(
  //       google::cloud::iam_v2::PoliciesLimitedErrorCountRetryPolicy(10).clone()));

Assuming you have created a custom idempotency policy. Such as:

 class CustomIdempotencyPolicy
    : public google::cloud::iam_v2::PoliciesConnectionIdempotencyPolicy {
 public:
  ~CustomIdempotencyPolicy() override = default;
  std::unique_ptr<google::cloud::iam_v2::PoliciesConnectionIdempotencyPolicy>
  clone() const override {
    return std::make_unique<CustomIdempotencyPolicy>(*this);
  }
  // Override inherited functions to define as needed.
};