Creates a new order template or updates an existing one (upsert). If templateId is omitted from the URL, one is auto-generated.

A template stores the same fields as an order (name, description, amount, currency, and an optional pay schedule),
but is inert: it is hidden from normal order flows, is never billed, and carries no status. A name is required
so the template can be identified in a picker.
For one-time (UNSCHEDULED) templates, use dueInDays to specify a due date relative to when the template is used.
Templates do not store customers or billing tokens — those are supplied when the template is instantiated.