**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
"api/v1/dags/TEST_DAG_ID/dagRuns?execution_date_lte=2020-06-13T18:00:00+00:00",
                [
                    "TEST_START_EXEC_DAY_10",
                    "TEST_START_EXEC_DAY_11",
                    "TEST_START_EXEC_DAY_12",
                    "TEST_START_EXEC_DAY_13",
                ],
            ),
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?execution_date_gte=2020-06-16T18:00:00+00:00",
                [
                    "TEST_START_EXEC_DAY_16",
                    "TEST_START_EXEC_DAY_17",
                    "TEST_START_EXEC_DAY_18",
                    "TEST_START_EXEC_DAY_19",
                ],
            ),
        ]
    )
    @provide_session
    def test_date_filters_gte_and_lte(self, url, expected_dag_run_ids, session):
        dagrun_models = self._create_dag_runs()
        session.add_all(dagrun_models)
        session.commit()
        response = self.client.get(url, environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        assert response.json["total_entries"] == len(expected_dag_run_ids)
        dag_run_ids = [dag_run["dag_run_id"] for dag_run in response.json["dag_runs"]]
        assert dag_run_ids == expected_dag_run_ids
    def _create_dag_runs(self):