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/fragments.tmpl

@@ -16,7 +16,7 @@
 
 {{/* Qualified table name */}}
 {{- define "qualified_table" -}}
-{{.SchemaName}}.{{.TableName}}
+{{qual_table_raw .SchemaName .TableName}}
 {{- end -}}
 
 {{/* Index method clause */}}