* Implement more methods:
* `get_cms_block`, `delete_cms_block`
* `create_coupon`, `update_coupon`, `get_coupon`, `delete_coupon`, `delete_coupons`, `delete_coupons_by_codes`
* `delete_customer_address`
* Add helpers `sku_exists`, `sku_was_bought`, `get_categories_under_root`
* `remove_product_from_category`: fix for SKUs that contain slashes
* Improve some type hints on attributes methods