**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
dates = [
            '2020-06-10T18:00:00+00:00',
            '2020-06-11T18:00:00+00:00',
            '2020-06-12T18:00:00+00:00',
            '2020-06-13T18:00:00+00:00',
            '2020-06-14T18:00:00+00:00',
            '2020-06-15T18:00:00Z',
            '2020-06-16T18:00:00Z',
            '2020-06-17T18:00:00Z',
            '2020-06-18T18:00:00Z',
            '2020-06-19T18:00:00Z',
        ]
        dag = DagModel(dag_id="TEST_DAG_ID")
        dag_runs = [
            DagRun(
                dag_id="TEST_DAG_ID",
                run_id="TEST_START_EXEC_DAY_1" + str(i),
                run_type=DagRunType.MANUAL,
                execution_date=timezone.parse(dates[i]),
                start_date=timezone.parse(dates[i]),
                external_trigger=True,
                state='success',