**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**


airflow/api_connexion/endpoints/dag_run_endpoint.py
post_body = dagrun_schema.load(request.json, session=session)
    except ValidationError as err:
        raise BadRequest(detail=str(err))
    logical_date = post_body["execution_date"]
    run_id = post_body["run_id"]
    dagrun_instance = (
        session.query(DagRun)
        .filter(
            DagRun.dag_id == dag_id,
            or_(DagRun.run_id == run_id, DagRun.execution_date == logical_date),
        )
        .first()
    )
    if not dagrun_instance:
        dag_run = current_app.dag_bag.get_dag(dag_id).create_dagrun(
            run_type=DagRunType.MANUAL,
            run_id=run_id,
            execution_date=logical_date,
            state=State.QUEUED,
            conf=post_body.get("conf"),
            external_trigger=True,
            dag_hash=current_app.dag_bag.dags_hash.get(dag_id),
        )
        return dagrun_schema.dump(dag_run)
    if dagrun_instance.execution_date == logical_date:
        raise AlreadyExists(
            detail=f"DAGRun with DAG ID: '{dag_id}' and DAGRun logical date: '{logical_date}' already exists"
        )
    raise AlreadyExists(detail=f"DAGRun with DAG ID: '{dag_id}' and DAGRun ID: '{run_id}' already exists")