Add C# API for speech enhancement GTCRN models (#1990)

scripts/dotnet/OfflineSpeechDenoiserModelConfig.cs

@@ -0,0 +1,27 @@
+/// Copyright (c)  2025  Xiaomi Corporation (authors: Fangjun Kuang)
+
+using System.Runtime.InteropServices;
+
+namespace SherpaOnnx
+{
+    [StructLayout(LayoutKind.Sequential)]
+    public struct OfflineSpeechDenoiserModelConfig
+    {
+        public OfflineSpeechDenoiserModelConfig()
+        {
+            Gtcrn = new OfflineSpeechDenoiserGtcrnModelConfig();
+            NumThreads = 1;
+            Debug = 0;
+            Provider = "cpu";
+        }
+
+        public OfflineSpeechDenoiserGtcrnModelConfig Gtcrn;
+
+        public int NumThreads;
+
+        public int Debug;
+
+        [MarshalAs(UnmanagedType.LPStr)]
+        public string Provider;
+    }
+}