feat(GODT-2799): SMTP Service

Refactor code to isolate the SMTP functionality in a dedicated SMTP
service for each user as discussed in the Bridge Service Architecture
RFC.

Some shared types have been moved from `user` to `usertypes` so that
they can be shared with Service and User Code.

Finally due to lack of recursive imports, the user data SMTP needs
access to is hidden behind an interface until the User Identity service
is implemented.

internal/services/smtp/errors.go

@@ -0,0 +1,23 @@
+// 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 "errors"
+
+var ErrInvalidRecipient = errors.New("invalid recipient")
+var ErrInvalidReturnPath = errors.New("invalid return path")