**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": "2020-11-11T08:25:56.939143+00:00",
            },
            environ_overrides={"REMOTE_USER": "test"},
        )
        assert response.status_code == 400
        assert response.json["title"] == "logical_date conflicts with execution_date"
        assert response.json["detail"] == (
            "'2020-11-11T08:25:56.939143+00:00' != '2020-11-10T08:25:56.939143+00:00'"
        )
    @parameterized.expand(
        [
            ({'execution_date': "2020-11-10T08:25:56.939143"}, 'Naive datetime is disallowed'),
            ({'execution_date': "2020-11-10T08:25:56P"}, "{'logical_date': ['Not a valid datetime.']}"),
            ({'logical_date': "2020-11-10T08:25:56.939143"}, 'Naive datetime is disallowed'),
            ({'logical_date': "2020-11-10T08:25:56P"}, "{'logical_date': ['Not a valid datetime.']}"),
        ]
    )
    def test_should_response_400_for_naive_datetime_and_bad_datetime(self, data, expected):