SupplierOrderNo in Purchase order contract (RTC-27574)

"SupplierOrderNo" is identifier of order, which is unique per supplier. "ExternalOrderNo" is globally unique identifier used in import. PurchaseOrder and OrderDespatchAdvice import and export contracts contain "SupplierOrderNo" value.
"Ext. Order No" column is added to "Supplier orders" grid in user interface.
"SupplierOrder" is renamed to "PurchaseOrder" in import/export contracts blob types and import documentation.
API for order info for item for given store is renamed to "PurchaseOrders" (URL and documentation).

Import of items (RTC-26800)

All item changes from Item Service are imported to Procurement database (SKU, MainGTIN, Brand, Name, status, ItemNo, assigned suppliers). Only items with status: Draft, Active, Stopped (not inactive and deleted) and delivered by selected supplier can be ordered, but it is possible to see and edit existing order lines containing incorrect items. When adding item through item search or 3rd party import, if item or item-supplier relation does not exist in Procurement database, placeholder row is created (values are expected to come in next import from Item Service). Order with placeholder store, supplier, item, item-supplier cannot be submitted.