Initial implementation of AccountSettings. (#6275)

* Make the UserPreference macro also generate an accessor for the key name.

* Initial implementation of AccountSettings.

- Add an AccountUserDefaults storage with a userID prefix on the key.
- Migrate the 2 values reset by AppSettings.resetSessionSpecificSettings to AccountSettings.

* Rename UserSettings.volatile() to UserSettings.mock()

Given it now has a userID parameter with a default value..

UnitTests/Sources/RoomSummaryProviderTests.swift

@@ -122,7 +122,7 @@ final class RoomSummaryProviderTests {
     // MARK: - Helpers
     
     private func setup(isLowPriorityFilterEnabled: Bool = false, includesOnlyJoinedRooms: Bool = false, setsRoomList: Bool = true) {
-        userSettings = UserSettings.volatile()
+        userSettings = UserSettings.mock()
         userSettings.lowPriorityFilterEnabled = isLowPriorityFilterEnabled
         
         let stateEventStringBuilder = RoomStateEventStringBuilder(userID: "@me:matrix.org")