feat: execute full-repo remaining parity closure plan

src/NATS.Server/NatsOptions.cs

@@ -40,6 +40,10 @@ public sealed class NatsOptions
     // Default/fallback
     public string? NoAuthUser { get; set; }
 
+    // Auth extensions
+    public Auth.ExternalAuthOptions? ExternalAuth { get; set; }
+    public Auth.ProxyAuthOptions? ProxyAuth { get; set; }
+
     // Auth timing
     public TimeSpan AuthTimeout { get; set; } = TimeSpan.FromSeconds(2);