Implement order/then_order_by for combination clauses (Union, …)

[Aethelflaed]

Postgres, mysql and sqlite all support ordering a combination query.

We could allow this by implementing OrderDsl / ThenOrderDsl for CombinationClause.

From what I understand, it would need to verify that the order's expr is valid for a given query source (as it's only implemented for SelectStatement<FromClause<F>, ...>).

It seems possible to mimic this constraint by implementing the traits only if the combinator's Source or Rhs is a query source (in which case the combinator itself becomes a query source)

I'm keen on working on that after #3084 if the idea is sound

[weiznich]

I fear that's currently not easily possible as the "subqueries" could have different select expressions with different field types. Making that possible would require a considerable amount of design work and likely some other features like alias! support for single fields first.

What should be relatively easy to implement is the fix for positional_order_by as that only requires changing what structure is generated.

[Aethelflaed]

I'll start with positional_order_by then

Edit: #4251