feat (feed): add new filters to personalize feed's products

[petitphp]

• add filter shopping_feed_products_for_feed to add or remove products from feed
• add filter shopping_feed_order_products_product_sku to modify SKU reference when importing and order from SF
• remove generator when formatting products

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Note

Medium Risk
Moderate risk because it changes the shape/type of product data passed into feed generation (from yielded arrays to Product objects) and updates generator mapper/filter signatures, which could break feed output if downstream expectations differ.

Overview
Adds a new customization point for feed generation by applying shopping_feed_products_for_feed to the formatted product list right before writing the XML, enabling third parties to add/remove items from the feed.

Adds shopping_feed_order_products_product_sku to let integrations rewrite the incoming ShoppingFeed order SKU before resolving WooCommerce products.

Refactors feed product formatting and generator callbacks to operate on ShoppingFeedWC\Products\Product objects directly (instead of arrays/wrappers), updating type hints/imports accordingly.

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