GraphQL: Add relatedToProducts/relatedToVariants RelationArgumentHandler

[denisyilmaz]

Craft Commerce’s GraphQL API currently doesn’t provide relatedToProducts / relatedToVariants relation handlers similar to EntryRelationCriteriaInput.

For example, we can currently do:

query GetRelatedProducts {
  products(relatedToEntries: [{slug: "some-article", withStructure: true}]) {
    title
  }
}

I’d like equivalent relation criteria for products and variants so it’s possible to query entries (or other elements) that are related to specific products or variants, and to express structure-aware relations (e.g. withStructure: true) in that context as well.

query GetArticle {
  entries(relatedToProducts: [{slug: "some-product", withStructure: true}]) {
    title
  }
}