**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()
        response = self.client.post(
            "api/v1/dags/TEST_DAG_ID/dagRuns",
            json={
                "dag_run_id": "TEST_DAG_RUN_ID_6",
                "execution_date": self.default_time,
            },
            environ_overrides={'REMOTE_USER': "test"},
        )
        assert response.status_code == 409, response.data
        assert response.json == {
            "detail": "DAGRun with DAG ID: 'TEST_DAG_ID' and "
            "DAGRun logical date: '2020-06-11 18:00:00+00:00' already exists",
            "status": 409,
            "title": "Conflict",
            "type": EXCEPTIONS_LINK_MAP[409],
        }
    def test_should_raises_401_unauthenticated(self):