lifecycle_publisher: method naming

Probably not the most important issue but I felt like inquiring about it :)

Is there a rationale behind naming the methods on_activate() and on_deactivate() with the "on" prefix in LifecyclePublisherInterface?

To me, it feels wrong to call these methods directly. They feel like state transition methods that should be called internally when a state transition is requested. Just like in LifecycleNodeInterface I would expect to have methods activate() and deactivate() which would then trigger the state transition.

This is probably loosely related to #654. A universal lifecycle interface for nodes and helper classes with automatic triggering of state transitions would be great!