Fetch PO template versions for the company. A version is the immutable snapshot of a template's schema: it owns the field definitions a purchase order created against it must comply with. To introspect the schema an integrator will write POs against, request the active version (status=active) and read its field definitions. To update a purchase order created with an older template, request the specific historical version by id (purchase_order.po_template_version_id) so the correct schema is used.