[Swift 6 Migration] Create a Sendable wrapper for UserDefaults

[data-sync-user]

We have a few tickets where types using UserDefaults can’t be Sendable due to a UserDefaults property.

We could implement a general Sendable UserDefaults wrapper which we use in these places.

A few examples:

• https://mozilla-hub.atlassian.net/browse/FXIOS-12597
• https://mozilla-hub.atlassian.net/browse/FXIOS-12589

Hint from Claude.ai: https://claude.ai/share/108766af-ab13-4d3d-b0e7-7b6983ad46d4

actor UserDefaultsManager {
    private let userDefaults = UserDefaults.standard
    
    func getString(forKey key: String) -> String? {
        userDefaults.string(forKey: key)
    }
    
    func setString(_ value: String, forKey key: String) {
        userDefaults.set(value, forKey: key)
    }
    
    func getBool(forKey key: String) -> Bool {
        userDefaults.bool(forKey: key)
    }
    
    func setBool(_ value: Bool, forKey key: String) {
        userDefaults.set(value, forKey: key)
    }
}

// Usage
let settingsManager = UserDefaultsManager()
await settingsManager.setString("value", forKey: "key")
let value = await settingsManager.getString(forKey: "key")

┆Issue is synchronized with this Jira Task