feat(GODT-2801): Identity Service

Identity Service contains all the information related to user state,
addresses and keys.

This patch also introduces the `State` type which can be used by other
services to maintain their own copy of this state to avoid lock
contention.

Finally, there are currently no external facing methods via a CPC
interface. Those will added as needed once the refactoring of the
architecture is complete.

internal/services/userevents/subscribable.go

@@ -0,0 +1,34 @@
+// 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 userevents
+
+// Subscribable represents a type that allows the registration of event subscribers.
+type Subscribable interface {
+	Subscribe(subscription Subscription)
+	Unsubscribe(subscription Subscription)
+}
+
+type NoOpSubscribable struct{}
+
+func (n NoOpSubscribable) Subscribe(_ Subscription) {
+	// Does nothing
+}
+
+func (n NoOpSubscribable) Unsubscribe(_ Subscription) {
+	// Does nothing
+}