Add keyword spotting for C# (#1105)

scripts/dotnet/KeywordResult.cs

@@ -0,0 +1,44 @@
+/// Copyright (c)  2024  Xiaomi Corporation
+
+using System;
+using System.Runtime.InteropServices;
+using System.Text;
+
+namespace SherpaOnnx
+{
+    public class KeywordResult
+    {
+        public KeywordResult(IntPtr handle)
+        {
+            Impl impl = (Impl)Marshal.PtrToStructure(handle, typeof(Impl));
+
+            // PtrToStringUTF8() requires .net standard 2.1
+            // _keyword = Marshal.PtrToStringUTF8(impl.Keyword);
+
+            int length = 0;
+
+            unsafe
+            {
+                byte* buffer = (byte*)impl.Keyword;
+                while (*buffer != 0)
+                {
+                    ++buffer;
+                    length += 1;
+                }
+            }
+
+            byte[] stringBuffer = new byte[length];
+            Marshal.Copy(impl.Keyword, stringBuffer, 0, length);
+            _keyword = Encoding.UTF8.GetString(stringBuffer);
+        }
+
+        [StructLayout(LayoutKind.Sequential)]
+        struct Impl
+        {
+            public IntPtr Keyword;
+        }
+
+        private String _keyword;
+        public String Keyword => _keyword;
+    }
+}