Checkpoint: NotificationCenter

Implement the NotificationCenter described in the checkpoint: ``csharp public sealed record NotificationEventArgs(string Id, string Channel, string Message); public class NotificationCenter { public event EventHandler<NotificationEventArgs>? Published; /* every accepted notification */ public event EventHandler<NotificationEventArgs>? Filtered; /* every rejected notification */ /* publishes: filter null or filter(msg)==true -> raise Published; otherwise raise Filtered. Ids are "n-1", "n-2", ... starting at 1 and incrementing for EVERY Publish call (accepted or not). */ public void Publish(string channel, string message, Predicate<string>? filter); /* clears both subscription lists */ public void UnsubscribeAll(); } ``