Add Flutter example for speaker identification (#894)

sherpa-onnx/flutter/example/lib/utils.dart

@@ -0,0 +1,18 @@
+// Copyright (c)  2024  Xiaomi Corporation
+import 'package:path/path.dart';
+import 'package:path_provider/path_provider.dart';
+import 'package:flutter/services.dart' show rootBundle;
+import "dart:io";
+
+// Copy the asset file from src to dst
+Future<String> copyAssetFile({required String src, required String dst}) async {
+  final Directory directory = await getApplicationDocumentsDirectory();
+  final target = join(directory.path, dst);
+
+  final data = await rootBundle.load(src);
+  final List<int> bytes =
+      data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
+  await File(target).writeAsBytes(bytes);
+
+  return target;
+}