Flutter Config toJson/fromJson (#1893)

flutter/sherpa_onnx/lib/src/feature_config.dart

@@ -3,11 +3,23 @@
 class FeatureConfig {
   const FeatureConfig({this.sampleRate = 16000, this.featureDim = 80});
 
+  factory FeatureConfig.fromJson(Map<String, dynamic> json) {
+    return FeatureConfig(
+      sampleRate: json['sampleRate'] as int? ?? 16000,
+      featureDim: json['featureDim'] as int? ?? 80,
+    );
+  }
+
   @override
   String toString() {
     return 'FeatureConfig(sampleRate: $sampleRate, featureDim: $featureDim)';
   }
 
+  Map<String, dynamic> toJson() => {
+        'sampleRate': sampleRate,
+        'featureDim': featureDim,
+      };
+
   final int sampleRate;
   final int featureDim;
 }