Warn about unused result of add_on_set_parameters_callback (#1238) (#1244)

If the user doesn't retain a reference to the returned shared pointer there will be zero references and their callback will be unregistered.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

rclcpp_lifecycle/include/rclcpp_lifecycle/lifecycle_node.hpp

@@ -42,6 +42,8 @@
 #include <utility>
 #include <vector>
 
+#include "rcutils/macros.h"
+
 #include "rcl/error_handling.h"
 #include "rcl/node.h"
 
@@ -456,6 +458,7 @@ public:
    * \sa rclcpp::Node::add_on_set_parameters_callback
    */
   RCLCPP_LIFECYCLE_PUBLIC
+  RCUTILS_WARN_UNUSED
   rclcpp_lifecycle::LifecycleNode::OnSetParametersCallbackHandle::SharedPtr
   add_on_set_parameters_callback(
     rclcpp_lifecycle::LifecycleNode::OnParametersSetCallbackType callback);