**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
session,
    dag_id,
    start_date_gte=None,
    start_date_lte=None,
    execution_date_gte=None,
    execution_date_lte=None,
    end_date_gte=None,
    end_date_lte=None,
    offset=None,
    limit=None,
    order_by='id',
):
    """Get all DAG Runs."""
    query = session.query(DagRun)