**Description**

It would be great having a option to abort a DAG Runs through the REST API.

**Use case / motivation**

The proposed input params would be:
 - DAG_ID
 - DAG_RUN_ID

The DAG Run should abort all its tasks running and mark them as "failed".

**Are you willing to submit a PR?**

**Related Issues**


tests/api_connexion/endpoints/test_dag_run_endpoint.py
default_time = "2020-06-11T18:00:00+00:00"
    default_time_2 = "2020-06-12T18:00:00+00:00"
    default_time_3 = "2020-06-13T18:00:00+00:00"
    @pytest.fixture(autouse=True)
    def setup_attrs(self, configured_app) -> None:
        self.app = configured_app
        self.client = self.app.test_client()  
        clear_db_runs()
        clear_db_dags()
    def teardown_method(self) -> None:
        clear_db_runs()
        clear_db_dags()
    def _create_dag(self, dag_id):
        dag_instance = DagModel(dag_id=dag_id)
        with create_session() as session:
            session.add(dag_instance)
        dag = DAG(dag_id=dag_id, schedule_interval=None)
        self.app.dag_bag.bag_dag(dag, root_dag=dag)
    def _create_test_dag_run(self, state='running', extra_dag=False, commit=True):