Batch assign/unassign tier structures

<Warning>

<Badge color="yellow">BETA endpoint</Badge>

This is a work-in-progress documentation of a BETA endpoint. The parameters, fields, request and response bodies, and other data may subject to change. If you want to share feedback or improvements, contact Voucherify support or your Technical Account Manager.

</Warning>

Assigns and/or unassigns tier structures to/from a program in a single batch. Unassign operations are processed before assign operations. Unassigning is only allowed while the program is in DRAFT status - otherwise the request is rejected with 423 Locked (key non_draft_program). A program can have at most one tier structure assigned. In strict mode (default) missing tier structures, inactive tier structures or missing assignments cause the whole batch to fail.