DDLStringVisitor should exclude dangling foreign keys

For resolved metadata we should omit foreign keys that reference tables that are filtered out.

For example with

CREATE FOREIGN TABLE G1(g1e1 integer, g1e2 varchar, PRIMARY KEY(g1e1, g1e2));
CREATE FOREIGN TABLE G2( g2e1 integer, g2e2 varchar, PRIMARY KEY(g2e1, g2e2), FOREIGN KEY (g2e1, g2e2) REFERENCES G1));

and a filter to only include G2, we should omit the foreign key so that the returned ddl is valid.