Add UDF for self-driving functions

[linmagit]

We are now leveraging the settings manager to invoke self-driving related functionalities (e.g., pilot_planning, train_forecast_model, and clear_query_cache). That's not what exactly the setting manager is designed for. Instead, we may want to add UDFs/built-in functions that can invoke these functionalities. For example, we can add a self_driving_action(action_enum) function, and use functions like select self_driving_action('clear_query_cache') to perform those behavior.