Support for ProductCollectionSortKeys and ProductImageSortKeys

[jc745]

Adding support for ProductCollectionSortKeys and ProductImageSortKeys would be welcome additions.

ProductCollectionSortKeys

You can imagine a situation where filtering collections of products on the frontend becomes much more feasible using ProductCollectionSortKeys (mentioned in gatsbyjs/gatsby-source-shopify#112).

ProductImageSortKeys

In (limited) testing it seems that Shopify's Admin API returns product images out of their natural order, whereas the Storefront API returns them in their natural order, based on how the product's images are positioned in the Shopify Admin UI.

Implementing ProductImageSortKeys would allow for proper ordering when using the Admin API, though I think defaulting to the POSITION value would be sufficient?

Example queries:

ProductCollectionSortKeys

query {
  collectionByHandle(handle: "classic"){
    products(first: 10, sortKey: BEST_SELLING) {
      ...
    }
  }
}

ProductImageSortKeys

query {
  productByHandle(handle: "snare-boot") {
    images(sortKey: POSITION) {
      ...
    }
  }
}