Add support for quantity in Kaui

[reshmabidikar]

Kill Bill 0.24.x added a new feature that allows specifying a quantity with a subscription. See killbill/killbill#1651 and https://killbill.github.io/slate/#subscription-subscription-resource. The corresponding changes need to be implemented on the Kaui side.

[vishalaug1]

Hi, the document link you shared doesn't have the quantity based pricing info. Can you please point me to the right document. Is the method updated to support quantity.