feat: implement /subz endpoint with account filter, test subject, and pagination

src/NATS.Server/NatsServer.cs

@@ -64,6 +64,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public Action? ReOpenLogFile { get; set; }
     public IEnumerable<NatsClient> GetClients() => _clients.Values;
 
+    public IEnumerable<Auth.Account> GetAccounts() => _accounts.Values;
+
     public Task WaitForReadyAsync() => _listeningStarted.Task;
 
     public void WaitForShutdown() => _shutdownComplete.Task.GetAwaiter().GetResult();