Updated the tools to use SKU fields instead of storing a GUID in metafields

Default SKU format is currently WC-X-######, where X is a single character representing the format and the number is a 6-digit number equivalent to 54040 + ConsignedItem.id

I've also added a field for barcode, but haven't done anything with it. It's up to you whether you want to hook up the UPC/ISBN of a physical product to this field. (There's a spot for it in Shopify... so it might be good to track it)