#40: Add UserSessionStoreProtocol.

Only log out of the specific account.
Add tests for the keychain controller.
Expand test coverage.
PR comments

ElementX/Sources/Services/UserSessionStore/KeychainControllerProtocol.swift

@@ -0,0 +1,17 @@
+//
+//  KeychainControllerProtocol.swift
+//  ElementX
+//
+//  Created by Stefan Ceriu on 14.02.2022.
+//  Copyright © 2022 Element. All rights reserved.
+//
+
+import Foundation
+
+protocol KeychainControllerProtocol {
+    func setAccessToken(_ accessToken: String, forUsername username: String)
+    func accessTokenForUsername(_ username: String) -> String?
+    func accessTokens() -> [(username: String, accessToken: String)]
+    func removeAccessTokenForUsername(_ username: String)
+    func removeAllAccessTokens()
+}