[DHL Parcel DE] Add option for label size in carrier settings

[MarcoWel]

Karrio is always requesting and returning the DHL Parcel DE labels in full A4 page format when requesting via UI. This format is unusable to print on common label printers.

The A4 format is hardcoded in karrio for both system label types "PDF" and "ZPL":

karrio/modules/connectors/dhl_parcel_de/karrio/providers/dhl_parcel_de/units.py

Line 50 in c3c7ffc

PDF_A4 = ("PDF", "A4")

DHL supports other label types such as PDF_910_300_700, but I found no way in karrio GUI to change the label type accordingly.
This is a crucial feature, please add a way to specify label size in the carrier settings.

It can be done by setting label_type in karrio API:
https://docs.karrio.io/reference/api/create-shipment