Identifier support added to the productUpdate mutation

The productUpdate mutation now accepts an identifier argument, allowing you to look up the product to update by id, handle, or customId instead of passing the product ID inside the input.

The id and handle identifiers let you specify a product's global ID or handle directly. The customId identifier lets you reference a product by a unique metafield value using a namespace, key, and value combination, enabling workflows where external systems identify products by their own IDs.