Added customer and product metafields to order exports

Order exports now support additional metafield types, giving you richer context about customers, products, and variants without running separate exports.

🚀 What’s New

• Customer metafields – export metafields from the customer associated with each order.
• Product metafields – export metafields from products in order line items.
• Variant metafields – export metafields from variants in order line items.

🔧 Improvements

• Metafield column headers now show the owner type (Customer, Product, or Variant) for clarity when multiple metafield types are included.
• Empty metafield columns are included in exports to maintain consistent structure across files.