**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
self._create_test_dag_run('success')  
        response = self.client.post(
            "api/v1/dags/~/dagRuns/list", json=payload, 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"] if dag_run]
        assert dag_run_ids == expected_dag_run_ids
class TestPostDagRun(TestDagRunEndpoint):
    @pytest.mark.parametrize("logical_date_field_name", ["execution_date", "logical_date"])
    @pytest.mark.parametrize(
        "dag_run_id, logical_date",
        [
            pytest.param("TEST_DAG_RUN", "2020-06-11T18:00:00+00:00", id="both-present"),
            pytest.param(None, "2020-06-11T18:00:00+00:00", id="only-date"),
            pytest.param(None, None, id="both-missing"),
        ],
    )
    def test_should_respond_200(self, logical_date_field_name, dag_run_id, logical_date):
        self._create_dag("TEST_DAG_ID")
        
        
        fixed_now = timezone.utcnow()
        request_json = {}
        if logical_date is not None: