Add addFieldIdForResolving or customFields option

[DobryninVyacheslav]

Is your feature request related to a problem? Please describe.
Let's say we have the following graphql schema (example taken from @thisarattr's question related to this issue):

type Query {
  book(id: ID): Book
}
type Book {
  name: String!
  author: Author
}
type Author {
  name: String!
  age: Int
}

The author should be resolved by a separate request, so I'm adding the following:

fieldsWithResolvers = setOf(
    "Book.author",
)

And get a Book model like:

data class Book(
  val name: String,
)

However, my database stores the author id directly in the book:

class BookEntity(
  val name: String,
  val authorId: String?,
)

So for me it looks more natural to pass id through Book for Author resolving than through GraphQLContext. And, as I understand it, for the author of this comment too.

Describe the solution you'd like
Thus would like to have a model like:

data class Book(
  val name: String,
  val authorId: String?,
)

Maybe it could be the addFieldIdForResolving option.

Describe alternatives you've considered
Or maybe it's better to be able to just set any custom fields (for example, option customFields with mapOf("Book.authorId" to "String") value).

[kobylynskyi]

I like more an idea of introducing customFields, because it provides more flexibility (not just Id fields).