Support `UNION (ALL) BY NAME` syntax (duckdb, spark)

This is a very useful form of `UNION` that has the backend align the columns on either side of the union: https://duckdb.org/docs/sql/query_syntax/setops.html#union-all-by-name.

Would be wonderful to have parsing support :)
Many thanks!

---
```sql
SELECT * FROM table_x UNION BY NAME SELECT * FROM table_y;
```