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.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.10%. Comparing base (560a13c) to head (0497ea7).

Files with missing lines	Patch %	Lines
src/parser/mod.rs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2492   +/-   ##
=======================================
  Coverage   81.09%   81.10%           
=======================================
  Files          42       42           
  Lines       33684    33694   +10     
  Branches    33684    33694   +10     
=======================================
+ Hits        27316    27326   +10     
+ Misses       2798     2797    -1     
- Partials     3570     3571    +1     

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