Postgres: Support aliases for JOIN USING

[BenSatori]

Support PostgreSQL's optional alias on JOIN USING columns, e.g. JOIN t2 USING (id) AS joined_cols.

PostgreSQL documents this as USING ( join_column [, ...] ) [ AS join_using_alias ]: https://www.postgresql.org/docs/current/sql-select.html

Implementation notes:

• Preserve the existing JoinConstraint::Using(Vec<ObjectName>) variant for joins without aliases.
• Add JoinConstraint::UsingAlias(Vec<ObjectName>, Ident) for the alias form.
• Add a minimal PostgreSQL regression test.

AI assistance was used to prepare this change.