title: Airflow - How to perform callbacks on a DAG level
body: <p>I have a usecase in Airflow where I need to override the DAG class to create my own type of DAGs. What i require is, whenever my DAG is called i need to execute a certain function. I know there is <code>on_success_callback</code> and <code>on_failure_callback</code> but those are for individual tasks. What i require is a callback on a DAG level. That is on successful instantiation of the DAG, I require this function to be executed regardless of tasks states. How can i do this in Airflow 2.2.</p>