* Change: Allow private properties on the WooCommerceGpfFeedItem object to be accessed read-only.
* Change: Pass WC_Product to woocommerce_gpf_feed_item filter
* Fix: GTIN in Bing feed was always blank
* Fix: Structured data contains incorrect SKU value
* Fix: Upgrade feed item cache bucket size to fix 'Opening and ending tag mismatch' errors with large variation output