**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
dag_runs = []
        dags = [DagModel(dag_id="TEST_DAG_ID")]
        dagrun_model_1 = DagRun(
            dag_id="TEST_DAG_ID",
            run_id="TEST_DAG_RUN_ID_1",
            run_type=DagRunType.MANUAL,
            execution_date=timezone.parse(self.default_time),
            start_date=timezone.parse(self.default_time),
            external_trigger=True,
            state=state,
        )
        dag_runs.append(dagrun_model_1)
        dagrun_model_2 = DagRun(
            dag_id="TEST_DAG_ID",
            run_id="TEST_DAG_RUN_ID_2",
            run_type=DagRunType.MANUAL,
            execution_date=timezone.parse(self.default_time_2),
            start_date=timezone.parse(self.default_time),
            external_trigger=True,
            state=state,
        )
        dag_runs.append(dagrun_model_2)
        if extra_dag:
            for i in range(3, 5):
                dags.append(DagModel(dag_id='TEST_DAG_ID_' + str(i)))