**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
if dag_id == "~":
        appbuilder = current_app.appbuilder
        query = query.filter(DagRun.dag_id.in_(appbuilder.sm.get_readable_dag_ids(g.user)))
    else:
        query = query.filter(DagRun.dag_id == dag_id)
    dag_run, total_entries = _fetch_dag_runs(
        query,
        end_date_gte,
        end_date_lte,
        execution_date_gte,
        execution_date_lte,
        start_date_gte,
        start_date_lte,
        limit,
        offset,
        order_by,
    )
    return dagrun_collection_schema.dump(DAGRunCollection(dag_runs=dag_run, total_entries=total_entries))
def _fetch_dag_runs(
    query,
    end_date_gte,
    end_date_lte,
    execution_date_gte,
    execution_date_lte,
    start_date_gte,
    start_date_lte,
    limit,
    offset,
    order_by,