Fix credential event callback reference lifetime

src/Lithnet.CredentialProvider.UnitTests.x64/ComInterop/RawCredentialEventBitmapBufferDelegate.cs

@@ -0,0 +1,10 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Lithnet.CredentialProvider.UnitTests.ComInterop
+{
+    // The trailing returnValue pointer characterizes the current CLR projection without PreserveSig.
+    // It does not establish the native ABI of the undocumented Windows Events3 interface.
+    [UnmanagedFunctionPointer(CallingConvention.StdCall)]
+    internal delegate int RawCredentialEventBitmapBufferDelegate(IntPtr instance, IntPtr credential, uint fieldId, uint imageBufferSize, IntPtr imageBuffer, IntPtr returnValue);
+}