#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/KeychainController.swift

@@ -0,0 +1,61 @@
+//
+//  KeychainController.swift
+//  ElementX
+//
+//  Created by Stefan Ceriu on 14.02.2022.
+//  Copyright © 2022 Element. All rights reserved.
+//
+
+import Foundation
+import KeychainAccess
+
+class KeychainController: KeychainControllerProtocol {
+    private let keychain: Keychain
+    
+    init(identifier: String) {
+        keychain = Keychain(service: identifier)
+    }
+ 
+    func setAccessToken(_ accessToken: String, forUsername username: String) {
+        do {
+            try keychain.set(accessToken, key: username)
+        } catch {
+            MXLog.error("Failed storing user access token with error: \(error)")
+        }
+    }
+    
+    func accessTokenForUsername(_ username: String) -> String? {
+        do {
+            return try keychain.get(username)
+        } catch {
+            MXLog.error("Failed retrieving user access token")
+            return nil
+        }
+    }
+    
+    func accessTokens() -> [(username: String, accessToken: String)] {
+        keychain.allKeys().compactMap { username in
+            guard let accessToken = accessTokenForUsername(username) else {
+                return nil
+            }
+            
+            return (username, accessToken)
+        }
+    }
+    
+    func removeAccessTokenForUsername(_ username: String) {
+        do {
+            try keychain.remove(username)
+        } catch {
+            MXLog.error("Failed removing access token with error: \(error)")
+        }
+    }
+    
+    func removeAllAccessTokens() {
+        do {
+            try keychain.removeAll()
+        } catch {
+            MXLog.error("Failed removing all tokens")
+        }
+    }
+}