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..

ElementX/Sources/Application/Settings/Storage/UserDefaultsProtocol.swift

@@ -0,0 +1,82 @@
+//
+// Copyright 2025 Element Creations Ltd.
+// Copyright 2022-2025 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+import Foundation
+
+nonisolated protocol UserDefaultsProtocol: AnyObject {
+    func data(forKey key: String) -> Data?
+    func object(forKey key: String) -> Any?
+    func removeObject(forKey key: String)
+    func set(_ value: Any?, forKey key: String)
+    
+    func reset()
+    func dictionaryRepresentation() -> [String: Any]
+}
+
+// MARK: - Codable handling
+
+nonisolated extension UserDefaultsProtocol {
+    /// Reads and writes a `Codable` value stored under `key`, used by the `@UserPreference` macro.
+    ///
+    /// When `Value` conforms to `PlistRepresentable` the Codable encode/decode phase is skipped and
+    /// the value is stored natively in the plist.
+    subscript<Value: Codable>(key: String) -> Value? {
+        get {
+            if Value.self is PlistRepresentable.Type {
+                object(forKey: key) as? Value
+            } else {
+                data(forKey: key).flatMap { try? JSONDecoder().decode(Value.self, from: $0) }
+            }
+        }
+        set {
+            // Detects correctly double optionals like this: String?? = .some(nil)
+            if newValue.isNil {
+                removeObject(forKey: key)
+            } else if Value.self is PlistRepresentable.Type {
+                set(newValue, forKey: key)
+            } else {
+                set(try? JSONEncoder().encode(newValue), forKey: key)
+            }
+        }
+    }
+}
+
+private nonisolated protocol Nullable {
+    var isNil: Bool { get }
+}
+
+nonisolated extension Optional: Nullable {
+    var isNil: Bool {
+        switch self {
+        case .none:
+            return true
+        case .some(let nullable as Nullable):
+            return nullable.isNil
+        case .some:
+            return false
+        }
+    }
+}
+
+// MARK: - PlistRepresentable
+
+/// A protocol to mark types as being plist compliant.
+/// The `UserDefaultsProtocol` storage subscript uses this to avoid encoding/decoding plist compliant values with Codable.
+nonisolated protocol PlistRepresentable { }
+
+extension Bool: PlistRepresentable { }
+extension String: PlistRepresentable { }
+extension Int: PlistRepresentable { }
+extension Float: PlistRepresentable { }
+extension Double: PlistRepresentable { }
+extension Date: PlistRepresentable { }
+extension Data: PlistRepresentable { }
+
+extension Array: PlistRepresentable where Element: PlistRepresentable { }
+extension Dictionary: PlistRepresentable where Key == String, Value: PlistRepresentable { }
+extension Optional: PlistRepresentable where Wrapped: PlistRepresentable { }