Add support for exporting discount codes

[d-beck]

It would be helpful if we could export a list of discount codes and price rules from the Shopify API. We can use the Matrixify format for reference.

References

• https://shopify.dev/docs/api/admin-rest/2023-04/resources/discountcode
• https://matrixify.app/documentation/discounts/
• Matrixify-Import-Demo-Discounts.xlsx