px4_work_queue: directly support SITL lockstep

- the purpose is to ensure that every WorkItem (and WorkItems scheduled
by WorkItems) is allowed to run to completion every step
 - per workqueue register a lockstep component whenever a work item is
added (if not already registered)
 - once the work queue is empty unregister component

platforms/common/include/px4_platform_common/px4_work_queue/WorkQueue.hpp

@@ -100,6 +100,10 @@ private:
 	BlockingList<WorkItem *>	_work_items;
 	px4::atomic_bool		_should_exit{false};
 
+#if defined(ENABLE_LOCKSTEP_SCHEDULER)
+	int _lockstep_component {-1};
+#endif // ENABLE_LOCKSTEP_SCHEDULER
+
 };
 
 } // namespace px4