GET maintenance/work-orders: Add work order external ID

The GET maintenance/work-orders endpoint has some new features! Added an optional externalId string to the work order response holding the identifier assigned by a third-party system when the work order was imported (for example a Fleetio or Eptura work order ID). The field is returned only when the request sets includeExternalIds=true and the work order has an external ID, and only for organizations with this field enabled.