feat(relations): 🎉 add flatten schema option for output

* Introduce `--flatten-schema` flag to convert, merge, and split commands.
* Modify database writing functions to support flattened schema names.
* Update template functions to handle schema.table naming convention.
* Enhance PostgreSQL writer to utilize flattened schema in generated SQL.
* Update tests to ensure compatibility with new flattening feature.
* Dependencies updated for improved functionality.

pkg/writers/pgsql/templates/drop_index.tmpl

@@ -1 +1 @@
-DROP INDEX IF EXISTS {{quote_ident .SchemaName}}.{{quote_ident .IndexName}} CASCADE;
+DROP INDEX IF EXISTS {{qual_table .SchemaName .IndexName}} CASCADE;