Declarative cache doesn't work with unions

[martinbonnin]

Description

Follow up from this slack discussion.

type User @typePolicy(keyFields: "id") {
  id 
  name
}

type UserError {
  code
}

union UserResult = UserError | User

type Query {
  getUser(id: ID!): UserResult
}

The UserResult type doesn't have keyfields and therefore this branch is not taken.

More generally, union result types are awkward to work with the cache. See also apollographql/apollo-kotlin#3827