feat: attach PostgreSQL constraints to existing indexes

[minleejae]

In POSTGRESQL mode, parse ALTER TABLE t ADD [CONSTRAINT name] UNIQUE|PRIMARY KEY USING INDEX index_name, including deferrability attributes and combined ALTER actions.

Expose this as ConstraintUsingIndex, with the new constraint name separated from getExistingIndexName(). Callers can inspect, edit or construct the reference; rendering is shared by toString() and the statement deparser. Metadata validation distinguishes the newly declared constraint from the existing index, and table discovery does not report the index as a table. Includes API usage documentation and regressions for ordinary constraints and Oracle's existing USING INDEX tail.

Validation: full Gradle check on Java 17; four fixtures executed in original and rendered form on PostgreSQL 18, checking pg_constraint for the resulting name, kind, attached index and deferrability.

Syntax reference: https://www.postgresql.org/docs/18/sql-altertable.html