chore(GODT-2799): Move Identifier interface to separate module

Required so we can move the smtp backend into the smtp service module.

internal/identifier/identifier.go

@@ -0,0 +1,32 @@
+// 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 identifier
+
+type Identifier interface {
+	GetUserAgent() string
+	HasClient() bool
+	SetClient(name, version string)
+	SetPlatform(platform string)
+	SetClientString(client string)
+	GetClientString() string
+}
+
+type UserAgentUpdater interface {
+	Identifier
+	SetUserAgent(name, version string)
+}