Add C# API for Dolphin CTC models (#2089)

scripts/dotnet/OfflineDolphinModelConfig.cs

@@ -0,0 +1,17 @@
+/// Copyright (c)  2025  Xiaomi Corporation (authors: Fangjun Kuang)
+
+using System.Runtime.InteropServices;
+
+namespace SherpaOnnx
+{
+    [StructLayout(LayoutKind.Sequential)]
+    public struct OfflineDolphinModelConfig
+    {
+        public OfflineDolphinModelConfig()
+        {
+            Model = "";
+        }
+        [MarshalAs(UnmanagedType.LPStr)]
+        public string Model;
+    }
+}