Add Support for Package Qty

[MarcoWel]

Moving this issue over from the karrio API because it is a UI feature, not an API feature.
karrioapi/karrio#351

Is your feature request related to a problem? Please describe.
Currently, having large multi-piece shipments with karrio is tedious to enter. Imagine a shipment of 20 boxes 10kg each with the same contents.

Describe the solution you'd like
We would like to specify a qty field for quickly adding identical packages to a shipments.

Describe alternatives you've considered
None.

Additional context
Getting and comparing rates for a 300 kg shipment with a max. weight of e.g. 30kg per parcel currently requires adding 10 packages to the rate request instead of one package item with qty 10.