Sort nodes alphabetically when generating selection expressions

[Cyberboss]

The following query will create two branches in a data loader when used with QueryContext<> and IDataLoader.With() and thus result in two identical database queries being executed despite both selecting the exact same columns of the exact same entity.

query TestQuery {
  usersById(id: "VXNlcjoxMDAwMg==") {
    name
    enabled
  }
  node(id: "VXNlcjoxMDAwMg==") {
    ... on User {
      enabled
      name
    }
  }
}

This is purely because the order of the assignments differ in the MemberInit expressions when the ISelection is converted to an Expression<Func<TResult, TResult>>. This change ensures fields are always processed alphabetically to minimize hash differences where possible.

Made these changes exclusively against v15. I assume they will be merged into main somehow?

[michaelstaib]

hey @Cyberboss can you redo it against main ... we always fix main than cherry pick it onto main-version-15.

[michaelstaib]

Needs to be retargeted to main.