Order by aggregated values

[fredcarle]

type User {
  name: String
  books: Int
  dvds: Int
  posters: Int
}

It would be useful for example to aggregate the stats and order by that aggregated value.

query {
   User(order: {total: DESC}) {
    total: _sum(books, dvds, posters)
  }
}

Given GQL specs, we may need to do something like this for GQL client to support it:

query {
   User(order: {_alias("total"): DESC}) {
    total: _sum(books, dvds, posters)
  }
}

[AndrewSisley]

This is part/a-duplicate of https://github.com/sourcenetwork/defradb/issues/681