Wrap offline ASR APIs to dart (#961)

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

@@ -0,0 +1,13 @@
+// Copyright (c)  2024  Xiaomi Corporation
+
+class FeatureConfig {
+  const FeatureConfig({this.sampleRate = 16000, this.featureDim = 80});
+
+  @override
+  String toString() {
+    return 'FeatureConfig(sampleRate: $sampleRate, featureDim: $featureDim)';
+  }
+
+  final int sampleRate;
+  final int featureDim;
+}