feat: add mqtt config model and parser for all Go MQTTOpts fields

src/NATS.Server/NatsOptions.cs

@@ -115,6 +115,9 @@ public sealed class NatsOptions
     // Subject mapping / transforms (source pattern -> destination template)
     public Dictionary<string, string>? SubjectMappings { get; set; }
 
+    // MQTT configuration (parsed from config, no listener yet)
+    public MqttOptions? Mqtt { get; set; }
+
     public bool HasTls => TlsCert != null && TlsKey != null;
 
     // WebSocket