Add Flutter example for speaker identification (#894)

sherpa-onnx/flutter/lib/src/wave_reader.dart

@@ -0,0 +1,32 @@
+// Copyright (c)  2024  Xiaomi Corporation
+import 'dart:ffi';
+import 'dart:typed_data';
+import 'package:ffi/ffi.dart';
+import "./sherpa_onnx_bindings.dart";
+
+class WaveData {
+  WaveData({required this.samples, required this.sampleRate});
+
+  /// normalized to [-1, 1]
+  Float32List samples;
+  int sampleRate;
+}
+
+WaveData readWave(String filename) {
+  final Pointer<Utf8> str = filename.toNativeUtf8();
+  Pointer<SherpaOnnxWave> wave =
+      SherpaOnnxBindings.readWave?.call(str) ?? nullptr;
+  calloc.free(str);
+
+  if (wave == nullptr) {
+    return WaveData(samples: Float32List(0), sampleRate: 0);
+  }
+
+  final samples = wave.ref.samples.asTypedList(wave.ref.numSamples);
+
+  final newSamples = Float32List.fromList(samples);
+  int sampleRate = wave.ref.sampleRate;
+  SherpaOnnxBindings.freeWave?.call(wave);
+
+  return WaveData(samples: newSamples, sampleRate: sampleRate);
+}