Add support for chaning passwords

MQTTnet.Core/Client/BaseMqttClientOptions.cs

@@ -3,20 +3,18 @@ using MQTTnet.Core.Serializer;
 
 namespace MQTTnet.Core.Client
 {
-    public abstract class BaseMqttClientOptions : IMqttClientOptions
+    public class MqttClientOptions : IMqttClientOptions
     {
         public MqttClientTlsOptions TlsOptions { get; set; } = new MqttClientTlsOptions();
 
         public MqttApplicationMessage WillMessage { get; set; }
 
-        public string UserName { get; set; }
-
-        public string Password { get; set; }
-
         public string ClientId { get; set; } = Guid.NewGuid().ToString("N");
 
         public bool CleanSession { get; set; } = true;
 
+        public IMqttClientCredentials Credentials { get; set; } = new MqttClientCredentials();
+
         public TimeSpan KeepAlivePeriod { get; set; } = TimeSpan.FromSeconds(5);
 
         public TimeSpan DefaultCommunicationTimeout { get; set; } = TimeSpan.FromSeconds(10);