Add ConnectAsync to PkceNoninteractiveLogin for async consistency

[petrsnd]

PkceNoninteractiveLogin.Connect() is sync-only but internally makes multiple HTTP calls to rSTS. Adding a ConnectAsync overload with CancellationToken support would:

• Allow callers in async contexts (ASP.NET, background services) to avoid blocking a thread pool thread
• Provide cancellation support for timeout scenarios
• Maintain API consistency with DeviceCodeLogin which offers both sync and async

This is lower priority than #227 since PkceNoninteractiveLogin is non-interactive (no human in the loop), so the blocking duration is short and predictable. Still a nice-to-have for async-native callers.