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/base_ddl.tmpl

@@ -11,7 +11,7 @@
 
 {{/* Base ALTER TABLE structure */}}
 {{- define "alter_table_base" -}}
-ALTER TABLE {{.SchemaName}}.{{.TableName}}
+ALTER TABLE {{qual_table_raw .SchemaName .TableName}}
   {{block "alter_operation" .}}{{end}};
 {{- end -}}
 
@@ -30,5 +30,5 @@ $$;
 
 {{/* Common drop pattern */}}
 {{- define "drop_if_exists" -}}
-{{block "drop_type" .}}{{end}} IF EXISTS {{.SchemaName}}.{{.ObjectName}};
+{{block "drop_type" .}}{{end}} IF EXISTS {{qual_table_raw .SchemaName .ObjectName}};
 {{- end -}}