Make SSPI auth mockable (#27036)

Before, the SSPI auth is only complied for Windows, it's difficult to
test and it breaks a lot.

Now, make the SSPI auth mockable and testable.

services/auth/sspiauth_windows.go

@@ -0,0 +1,19 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build windows
+
+package auth
+
+import (
+	"github.com/quasoft/websspi"
+)
+
+type SSPIUserInfo = websspi.UserInfo
+
+func sspiAuthInit() error {
+	var err error
+	config := websspi.NewConfig()
+	sspiAuth, err = websspi.New(config)
+	return err
+}