feat: add Account type with per-account SubList and client tracking

src/NATS.Server/Auth/Account.cs

@@ -0,0 +1,28 @@
+using System.Collections.Concurrent;
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Auth;
+
+public sealed class Account : IDisposable
+{
+    public const string GlobalAccountName = "$G";
+
+    public string Name { get; }
+    public SubList SubList { get; } = new();
+    public Permissions? DefaultPermissions { get; set; }
+
+    private readonly ConcurrentDictionary<ulong, byte> _clients = new();
+
+    public Account(string name)
+    {
+        Name = name;
+    }
+
+    public int ClientCount => _clients.Count;
+
+    public void AddClient(ulong clientId) => _clients[clientId] = 0;
+
+    public void RemoveClient(ulong clientId) => _clients.TryRemove(clientId, out _);
+
+    public void Dispose() => SubList.Dispose();
+}