spin until timeout

Feature request

I propose a spin until timeout function:

void spin_until_timeout(rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_ptr, 
                        std::chrono::duration< TimeRepT, TimeT > timeout)

This is useful for writing unit tests. Currently, I use spin_until_future_complete(), with a placeholder future, based on code in benchmark_service.cpp

constexpr char empty_service_name[] = "empty_service";
auto client = node->create_client<test_msgs::srv::Empty>(empty_service_name);
auto shared_request = std::make_shared<test_msgs::srv::Empty::Request>();
auto future = client->async_send_request(shared_request);
spin_until_future_complete(node, future, std::chrono::seconds(1));

Start with benchmark_service.cpp and the spin_until_future_complete entry point shown in the issue to understand the current placeholder-future pattern. Define completion as providing a spin_until_timeout API that accepts the node and duration directly, so unit tests no longer need a placeholder future.