feat: support PostgreSQL referential action column lists

[minleejae]

Support PostgreSQL ON DELETE SET NULL/SET DEFAULT (columns) as structured referential-action metadata. This is needed for composite foreign keys where deleting a referenced row should change only part of the referencing key.

Extract one shared action production for the two optional ON clauses. Column REFERENCES and table foreign keys in CREATE/ALTER use it. Column lists on ON UPDATE and incompatible actions are rejected.

Validation:

• Full Gradle check passed (Java 17), including grammar ambiguity, unit tests, coverage, formatting, Checkstyle, PMD, and SpotBugs.
• On PostgreSQL 17.11, CREATE and ALTER foreign keys changed only the selected column after deleting the referenced row, for both SET NULL and SET DEFAULT.
• Added AST, mutation, and parse/deparse regression coverage.

Syntax reference: official documentation.

Additional version-boundary validation: the original, toString, and deparser SQL was executed on PostgreSQL 18.6, including the focused behavioral cases. Reviewed the PostgreSQL 18 grammar. Runtime version numbers above identify the tested binaries; this does not claim support for every statement in those DBMS versions.

[manticore-projects]

Thank you!