**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
dagrun_model = DagRun(
            dag_id="TEST_DAG_ID",
            run_id="TEST_DAG_RUN_ID",
            run_type=DagRunType.MANUAL,
            execution_date=timezone.parse(self.default_time),
            start_date=timezone.parse(self.default_time),
            external_trigger=True,
            state='running',
        )
        session.add(dagrun_model)
        session.commit()
        result = session.query(DagRun).all()
        assert len(result) == 1
        response = self.client.get(
            "api/v1/dags/TEST_DAG_ID/dagRuns/TEST_DAG_RUN_ID", environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 200
        expected_response = {
            'dag_id': 'TEST_DAG_ID',
            'dag_run_id': 'TEST_DAG_RUN_ID',
            'end_date': None,
            'state': 'running',
            'logical_date': self.default_time,
            'execution_date': self.default_time,
            'external_trigger': True,
            'start_date': self.default_time,
            'conf': {},
        }
        assert response.json == expected_response
    def test_should_respond_404(self):