Refactor subscribe and unsubscribe to support MQTTv5.

Source/MQTTnet/Client/Unsubscribing/MqttClientUnsubscribeOptions.cs

@@ -0,0 +1,12 @@
+using System.Collections.Generic;
+using MQTTnet.Packets;
+
+namespace MQTTnet.Client.Unsubscribing
+{
+    public class MqttClientUnsubscribeOptions
+    {
+        public List<string> TopicFilters { get; set; } = new List<string>();
+
+        public List<MqttUserProperty> UserProperties { get; set; } = new List<MqttUserProperty>();
+    }
+}