add option to set DAG level Dataset outlets

### Description add a new argument `outlets` to the DAG constructor with the same effect as the BaseOperator's outlets argument when the full dag run is successfull ### Use case/motivation use case: I'm currently maintaining several DAGs with "Dataset Scheduling", where each DAG handles a specific dataset and declares that it is ready for downstream DAGs by an EmptyOperator with an `outlets` value this means I need to fan-in all the leafs of each DAG:  suggestion: add an `outlets` argument to the `DAG` constructor to mark the Dataset(s) as ready when the dag run is successful ### Related issues _No response_ ### Are you willing to submit a PR? - [X] Yes I am willing to submit a PR! ### Code of Conduct - [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)