Add cash on delivery

Hi,
I added a generic service and the first real usecase "cash on delivery". (See DHL documentation: [createShipmentDD/ShipmentOrder.Shipment.ShipmentDetails.Service.ServiceGroupOther.COD #2.2.1.11.6.2.](https://entwickler.dhl.de/group/ep/webservices/intraship/entwicklung-und-test/version-1.8/io-referenz/createshipmentdd/input#aui_3_2_0_1104))
It has been used for more than 4 months in my shop (production) and works flawlessly since then.

Best regards,
Maximilian