**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=data, environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 400
        assert response.json['detail'] == expected
    def test_response_404(self):
        response = self.client.post(
            "api/v1/dags/TEST_DAG_ID/dagRuns",
            json={"dag_run_id": "TEST_DAG_RUN", "execution_date": self.default_time},
            environ_overrides={'REMOTE_USER': "test"},
        )
        assert response.status_code == 404
        assert {
            "detail": "DAG with dag_id: 'TEST_DAG_ID' not found",
            "status": 404,
            "title": "DAG not found",
            "type": EXCEPTIONS_LINK_MAP[404],
        } == response.json