**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
    @parameterized.expand(
        [
            (
                {
                    "dag_run_id": "TEST_DAG_RUN",
                    "execution_date": "2020-06-11T18:00:00+00:00",
                    "conf": "some string",
                },
                "'some string' is not of type 'object' - 'conf'",
            )
        ]
    )
    def test_should_response_400_for_non_dict_dagrun_conf(self, data, expected):