Move extension to its own file

MastodonSDK/Sources/MastodonCore/AuthenticationServiceProvider.swift

@@ -124,13 +124,3 @@ private extension AuthenticationServiceProvider {
         }
     }
 }
-
-private extension UserDefaults {
-    @objc dynamic var didMigrateAuthentications: Bool {
-        get {
-            register(defaults: [#function: false])
-            return bool(forKey: #function)
-        }
-        set { self[#function] = newValue }
-    }
-}

MastodonSDK/Sources/MastodonCore/UserDefaults+Authentication.swift

@@ -0,0 +1,20 @@
+// Copyright © 2023 Mastodon gGmbH. All rights reserved.
+
+import Foundation
+
+public extension UserDefaults {
+
+    enum Keys {
+        static let didMigrateAuthenticationsKey = "didMigrateAuthentications"
+    }
+
+    @objc dynamic var didMigrateAuthentications: Bool {
+        get {
+            return bool(forKey: Keys.didMigrateAuthenticationsKey)
+        }
+
+        set {
+            set(newValue, forKey: Keys.didMigrateAuthenticationsKey)
+        }
+    }
+}