Divides one order into several child orders. The source order and its line items are superseded
once the split succeeds, and the associated cart is superseded along with them.

The source order must be in Draft, Pending, Confirmed or In Fulfillment status, and must not have
a custom plan associated with it.

The SplitType field selects how the order is divided, and determines which other field of the
request body is used:

• 'Split By Line Number' uses LineNumbers, where each inner list is one resulting order. Every
  line number supplied must exist on the source order, and may appear only once across the
  whole payload - listing one in two groups, or twice in the same group, is rejected with
  400 Bad Request.
• 'Split By Adhoc Criteria' uses Criteria, a list of line item property paths. Line items are
  grouped by the combined values of those properties, and each group becomes one order.
• 'Split By Predefined Criteria' uses CriteriaIds, referencing existing split criteria setups.
  Grouping must produce at least two groups, otherwise the request is rejected.

Any other SplitType value is rejected with 400 Bad Request.