refactor(schema): separate builder from domain object and improve API design

- Split SchemaDef into SchemaBuilder (parser) and SchemaDef (domain object)
- Change SchemaBuilder to be stateless and return SchemaDef instances
- Move table storage from builder to SchemaDef
- Add helper methods to constraint objects (contains, references, is_unique_on)
- Update method signatures to pass schema mutably to parsing methods
- Rename parse() to parse_sql() for clarity
- Remove utility methods from builder, moving logic to domain objects
- Update all tests to use new builder pattern

Signed-off-by: Svetlin Ralchev <iamralch@users.noreply.github.com>

Author: iamralch