Add Flutter example for speaker identification (#894)

sherpa-onnx/flutter/lib/sherpa_onnx.dart

@@ -0,0 +1,30 @@
+// Copyright (c)  2024  Xiaomi Corporation
+import 'dart:io';
+import 'dart:ffi';
+
+import 'src/sherpa_onnx_bindings.dart';
+export 'src/speaker_identification.dart';
+export 'src/online_stream.dart';
+export 'src/wave_reader.dart';
+
+final DynamicLibrary _dylib = () {
+  if (Platform.isIOS) {
+    throw UnsupportedError('Unknown platform: ${Platform.operatingSystem}');
+  }
+  if (Platform.isMacOS) {
+    return DynamicLibrary.open('libsherpa-onnx-c-api.dylib');
+  }
+  if (Platform.isAndroid || Platform.isLinux) {
+    return DynamicLibrary.open('libsherpa-onnx-c-api.so');
+  }
+
+  if (Platform.isWindows) {
+    return DynamicLibrary.open('sherpa-onnx-c-api.dll');
+  }
+
+  throw UnsupportedError('Unknown platform: ${Platform.operatingSystem}');
+}();
+
+void initBindings() {
+  SherpaOnnxBindings.init(_dylib);
+}