feat: share PostgreSQL foreign key enforcement attributes

[minleejae]

PostgreSQL 18 accepts ENFORCED and NOT ENFORCED on foreign keys, but JSqlParser currently rejects these clauses in table constraints and column REFERENCES definitions. This prevents consumers from inspecting or rewriting DDL that deliberately disables enforcement.

This change stores the optional enforcement flag in ConstraintAttributes and shares attribute parsing across CREATE/ALTER table constraints and column references. CHECK constraints delegate their existing enforcement accessors to the same state, avoiding duplicate storage and rendering. Column references stop before a following NOT NULL; NOT VALID remains a table-constraint form.

Validation:

• Full Gradle check with Java 17, including grammar ambiguity and static analysis.
• Regression tests cover AST mutation, omitted/true/false flags, attribute ordering, CREATE/ALTER, CHECK accessor compatibility, following column options, and malformed/duplicate clauses.
• Executed 22 accepted cases on PostgreSQL 18.6 in original, toString(), and StatementDeParser forms. Compared constraint catalogs and enforcement behavior, including deferred and NOT VALID constraints. Two invalid column-reference cases are rejected by both PostgreSQL and the parser.

Reference: PostgreSQL 18 CREATE TABLE.

[manticore-projects]

Thank you!