Add support for circular foreign keys

Currently SSG doesn't work with schemas with circular foreign keys (except maybe if they are in vocab tables?). There's conversation about this in https://github.com/alan-turing-institute/sqlsynthgen/issues/29

Start by reading the discussion in issue #29 to understand the proposed handling for circular foreign keys, then inspect the repository's existing schema and foreign-key processing entry points. Done means SSG can generate synthetic data for schemas containing circular foreign-key relationships, including the relevant regression coverage.

Written by the indexing model from the issue text.