Split mail sender sub package from mailer service package (#32618)

Move all mail sender related codes into a sub package of
services/mailer. Just move, no code change.
Then we just have dependencies on go-mail package in the new sub
package. We can use other package to replace it because it's
unmaintainable. ref #18664

services/mailer/sender/smtp_auth.go

@@ -0,0 +1,69 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package sender
+
+import (
+	"fmt"
+	"net/smtp"
+
+	"github.com/Azure/go-ntlmssp"
+)
+
+type loginAuth struct {
+	username, password string
+}
+
+// LoginAuth SMTP AUTH LOGIN Auth Handler
+func LoginAuth(username, password string) smtp.Auth {
+	return &loginAuth{username, password}
+}
+
+// Start start SMTP login auth
+func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+	return "LOGIN", []byte{}, nil
+}
+
+// Next next step of SMTP login auth
+func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		switch string(fromServer) {
+		case "Username:":
+			return []byte(a.username), nil
+		case "Password:":
+			return []byte(a.password), nil
+		default:
+			return nil, fmt.Errorf("unknown fromServer: %s", string(fromServer))
+		}
+	}
+	return nil, nil
+}
+
+type ntlmAuth struct {
+	username, password, domain string
+	domainNeeded               bool
+}
+
+// NtlmAuth SMTP AUTH NTLM Auth Handler
+func NtlmAuth(username, password string) smtp.Auth {
+	user, domain, domainNeeded := ntlmssp.GetDomain(username)
+	return &ntlmAuth{user, password, domain, domainNeeded}
+}
+
+// Start starts SMTP NTLM Auth
+func (a *ntlmAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+	negotiateMessage, err := ntlmssp.NewNegotiateMessage(a.domain, "")
+	return "NTLM", negotiateMessage, err
+}
+
+// Next next step of SMTP ntlm auth
+func (a *ntlmAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		if len(fromServer) == 0 {
+			return nil, fmt.Errorf("ntlm ChallengeMessage is empty")
+		}
+		authenticateMessage, err := ntlmssp.ProcessChallenge(fromServer, a.username, a.password, a.domainNeeded)
+		return authenticateMessage, err
+	}
+	return nil, nil
+}