add on_action(state, entity) to scenario_dag

[FredLL-Avaiga]

resolves #768

def on_dag_action(state: State, entity: Task | DataNode):
    print(f"on_dag_action(state, {entity})")

...

<|{scenario}|scenario_dag|on_action=on_dag_action|>

[FabienLelaquais]

I like that!