Add constructor fromPtr() for all flutter class with factory ctor. (#1667)

Co-authored-by: wangrui <wangrui@oecore.com>

flutter/sherpa_onnx/lib/src/tts.dart

@@ -82,6 +82,8 @@ class GeneratedAudio {
 }
 
 class OfflineTts {
+  OfflineTts.fromPtr({required this.ptr, required this.config});
+
   OfflineTts._({required this.ptr, required this.config});
 
   /// The user is responsible to call the OfflineTts.free()