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/usertypes/keys.go

@@ -0,0 +1,68 @@
+// 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 usertypes
+
+import (
+	"fmt"
+
+	"github.com/ProtonMail/go-proton-api"
+	"github.com/ProtonMail/gopenpgp/v2/crypto"
+	"github.com/sirupsen/logrus"
+)
+
+func WithAddrKR(apiUser proton.User, apiAddr proton.Address, keyPass []byte, fn func(userKR, addrKR *crypto.KeyRing) error) error {
+	userKR, err := apiUser.Keys.Unlock(keyPass, nil)
+	if err != nil {
+		return fmt.Errorf("failed to unlock user keys: %w", err)
+	}
+	defer userKR.ClearPrivateParams()
+
+	addrKR, err := apiAddr.Keys.Unlock(keyPass, userKR)
+	if err != nil {
+		return fmt.Errorf("failed to unlock address keys: %w", err)
+	}
+	defer addrKR.ClearPrivateParams()
+
+	return fn(userKR, addrKR)
+}
+
+func WithAddrKRs(apiUser proton.User, apiAddr map[string]proton.Address, keyPass []byte, fn func(*crypto.KeyRing, map[string]*crypto.KeyRing) error) error {
+	userKR, err := apiUser.Keys.Unlock(keyPass, nil)
+	if err != nil {
+		return fmt.Errorf("failed to unlock user keys: %w", err)
+	}
+	defer userKR.ClearPrivateParams()
+
+	addrKRs := make(map[string]*crypto.KeyRing, len(apiAddr))
+
+	for addrID, apiAddr := range apiAddr {
+		addrKR, err := apiAddr.Keys.Unlock(keyPass, userKR)
+		if err != nil {
+			return fmt.Errorf("failed to unlock address keys: %w", err)
+		}
+		defer addrKR.ClearPrivateParams()
+
+		if addrKR.CountDecryptionEntities() == 0 {
+			logrus.WithField("addressID", addrID).Warn("Address keyring has no decryption entities")
+		}
+
+		addrKRs[addrID] = addrKR
+	}
+
+	return fn(userKR, addrKRs)
+}