**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_dag("TEST_DAG_ID")
        response = self.client.post(
            "api/v1/dags/TEST_DAG_ID/dagRuns",
            json={
                "execution_date": "2020-11-10T08:25:56.939143+00:00",
                "logical_date": "2020-11-10T08:25:56.939143+00:00",
            },
            environ_overrides={"REMOTE_USER": "test"},
        )
        assert response.status_code == 200
        assert {
            "conf": {},
            "dag_id": "TEST_DAG_ID",
            "dag_run_id": "manual__2020-11-10T08:25:56.939143+00:00",
            "end_date": None,
            "execution_date": "2020-11-10T08:25:56.939143+00:00",
            "logical_date": "2020-11-10T08:25:56.939143+00:00",
            "external_trigger": True,
            "start_date": None,
            "state": "queued",
        } == response.json
    def test_should_response_400_for_conflicting_execution_date_logical_date(self):