PostgreSQL: Support PRIMARY KEY/UNIQUE USING INDEX

[guan404ming]

Why

PostgreSQL allows promoting an existing unique index to a PRIMARY KEY or UNIQUE table constraint via ALTER TABLE, which was
not supported by the parser.

How

• Add ConstraintUsingIndex variant to TableConstraint with display, span, and serialization support
• Parse USING INDEX index_name after PRIMARY KEY or UNIQUE keywords in constraint parsing
• Add tests for named/unnamed constraints and deferrable characteristics