**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
@parameterized.expand(
        [
            (
                "start_date in request json",
                "api/v1/dags/TEST_DAG_ID/dagRuns",
                {
                    "start_date": "2020-06-11T18:00:00+00:00",
                    "execution_date": "2020-06-12T18:00:00+00:00",
                },
                {
                    "detail": "Property is read-only - 'start_date'",
                    "status": 400,
                    "title": "Bad Request",
                    "type": EXCEPTIONS_LINK_MAP[400],
                },
            ),
            (
                "state in request json",
                "api/v1/dags/TEST_DAG_ID/dagRuns",
                {"state": "failed", "execution_date": "2020-06-12T18:00:00+00:00"},
                {
                    "detail": "Property is read-only - 'state'",
                    "status": 400,
                    "title": "Bad Request",
                    "type": EXCEPTIONS_LINK_MAP[400],
                },
            ),
        ]
    )
    def test_response_400(self, name, url, request_json, expected_response):