Enforce type coercion before children output deduplication in Union

Right now, query the following query produces plans that are not consistent over different underlying table providers. Query:

SELECT col1, col2, col3, NULLIF('','') AS col4
FROM table
UNION ALL
SELECT col2, col2, null AS col3, col4
FROM table;

This happens because of rule ordering:

• Sometimes: ... -> WidenSetOperationTypes -> ... -> ResolveReferences (deduplication of Union children outputs) -> ...
• Sometimes: ... -> ResolveReferences (deduplication of Union children outputs) -> ... -> WidenSetOperationTypes -> ...

In this issue I propose that we align those two by enforcing type coercion to happen before deduplication.