**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/schemas/dag_run_schema.py
"""The conf field"""
    def _serialize(self, value, attr, obj, **kwargs):
        if not value:
            return {}
        return json.loads(value) if isinstance(value, str) else value
    def _deserialize(self, value, attr, data, **kwargs):
        if isinstance(value, str):
            return json.loads(value)
        return value
_MISSING = object()
class DAGRunSchema(SQLAlchemySchema):
    """Schema for DAGRun"""
    class Meta:
        """Meta"""
        model = DagRun
        dateformat = "iso"
    run_id = auto_field(data_key='dag_run_id')
    dag_id = auto_field(dump_only=True)
    execution_date = auto_field(data_key="logical_date", validate=validate_istimezone)
    start_date = auto_field(dump_only=True)
    end_date = auto_field(dump_only=True)
    state = DagStateField(dump_only=True)
    external_trigger = auto_field(dump_default=True, dump_only=True)
    conf = ConfObject()
    @pre_load
    def autogenerate(self, data, **kwargs):