Optional externalThis property may be used to supply an external Service.Doorbell.
This is particularly handy when one is migrating from an existing implementation
to a DoorbellController and want to avoid loosing users automation by removing and deleting the service.
NOTE: You are responsible for managing the service yourself (e.g. creation, restoring, adding to accessory, ...)
Optional nameName used to for the Service.Doorbell service.
Options which are additionally supplied for a DoorbellController.