[Feature Request] ProductVariant does not expose the `barcode`

[martinfotp]

Hello 

We've added Schema.org markup to our headless storefront and Google recommends that we also expose the barcode (GTIN) value. From what I can see, right now there's no way to fetch this from the Storefront API. Is there any reason not expose this field? 

Right now, I've gone ahead and added in a metafields to expose the value but it would be great if this could also be exposed natively on the `ProductVariant` node. 

https://shopify.dev/docs/storefront-api/reference/products/productvariant