**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
"""Get list of DAG Runs"""
    body = request.get_json()
    try:
        data = dagruns_batch_form_schema.load(body)
    except ValidationError as err:
        raise BadRequest(detail=str(err.messages))
    appbuilder = current_app.appbuilder
    readable_dag_ids = appbuilder.sm.get_readable_dag_ids(g.user)
    query = session.query(DagRun)