Split shipment support for Kustom Shipping Assistant (KSA)

Brink now supports split shipments when using Kustom Shipping Assistant (KSA) as the checkout shipping integration.
What's new
Orders using KSA can be split into multiple deliveries, each with its own carrier, tracking, and capture amount.
Each split delivery is reported back to Kustom so the shopper sees accurate, per-parcel tracking in the Kustom app and post-purchase emails.
No breaking changes for merchants already on KSA: orders that ship in a single delivery continue to work exactly as before.