feat: add auth model types (User, NKeyUser, Permissions) and auth config to NatsOptions

src/NATS.Server/Auth/NKeyUser.cs

@@ -0,0 +1,9 @@
+namespace NATS.Server.Auth;
+
+public sealed class NKeyUser
+{
+    public required string Nkey { get; init; }
+    public Permissions? Permissions { get; init; }
+    public string? Account { get; init; }
+    public string? SigningKey { get; init; }
+}