Persist the useragent string across app starts. Make the class thread safe

[weibel]

This PR ensures the user agent is retrieved efficiently, stored persistently, and accessed in a thread-safe manner.

Key Features:

• Singleton Pattern: Ensures a single instance of UserAgentService is used throughout the application.
• Thread Safety: Utilizes a DispatchSemaphore to synchronize access to the user agent string.
• Persistent Storage: Stores the user agent string in UserDefaults, keyed by the current OS version, to minimize fetch time on subsequent app launches.
• User Agent Retrieval: Fetches the user agent string using a WKWebView, with an added delay to ensure the web view is ready for use.
• Extension for Persistence: Encapsulates the logic for loading and saving the user agent string from/to UserDefaults in an extension.

Implementation Details:

• The userAgent property provides thread-safe access to the user agent string.
• On initialization, the service attempts to load the user agent from UserDefaults. If not found, it fetches it using a WKWebView.
• The fetchUserAgent method includes a delay to ensure the WKWebView is ready and handles errors gracefully.
• An extension is used to manage persistence, including methods for loading, saving, and resetting the user agent string in UserDefaults.

[weibel]

The threading issue has been solved with prebid#989
The persistence is proposed in this prebid#992