feat(GODT-2799): SMTP service interacts directly with Server Manager

Bridge no longer needs to manually add and remove accounts from the
service.

internal/services/smtp/server_manager.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2023 Proton AG
+//
+// This file is part of Proton Mail Bridge.
+//
+// Proton Mail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Proton Mail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Proton Mail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package smtp
+
+import "context"
+
+type ServerManager interface {
+	AddSMTPAccount(ctx context.Context, service *Service) error
+	RemoveSMTPAccount(ctx context.Context, service *Service) error
+}
+
+type NullServerManager struct{}
+
+func NewNullServerManager() *NullServerManager {
+	return &NullServerManager{}
+}
+
+func (n NullServerManager) AddSMTPAccount(_ context.Context, _ *Service) error {
+	// Does nothing.
+	return nil
+}
+
+func (n NullServerManager) RemoveSMTPAccount(_ context.Context, _ *Service) error {
+	// Does nothing.
+	return nil
+}