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.

cmd/relspec/split.go

@@ -183,7 +183,8 @@ func runSplit(cmd *cobra.Command, args []string) error {
 		splitTargetType,
 		splitTargetPath,
 		splitPackageName,
-		"", // no schema filter for split
+		"",    // no schema filter for split
+		false, // no flatten-schema for split
 	)
 	if err != nil {
 		return fmt.Errorf("failed to write output: %w", err)