Added MqttClientUnsubscribeOptionsBuilder analogous to MqttClientSubscribeOptionsBuilder

Source/MQTTnet/Client/Unsubscribing/MqttClientUnsubscribeOptionsBuilder.cs

@@ -0,0 +1,60 @@
+using MQTTnet.Packets;
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace MQTTnet.Client.Unsubscribing
+{
+    public class MqttClientUnsubscribeOptionsBuilder
+    {
+        private readonly MqttClientUnsubscribeOptions _unsubscribeOptions = new MqttClientUnsubscribeOptions();
+
+        public MqttClientUnsubscribeOptionsBuilder WithUserProperty(string name, string value)
+        {
+            if (name is null) throw new ArgumentNullException(nameof(name));
+            if (value is null) throw new ArgumentNullException(nameof(value));
+
+            return WithUserProperty(new MqttUserProperty(name, value));
+        }
+
+        public MqttClientUnsubscribeOptionsBuilder WithUserProperty(MqttUserProperty userProperty)
+        {
+            if (userProperty is null) throw new ArgumentNullException(nameof(userProperty));
+
+            if (_unsubscribeOptions.UserProperties is null)
+            {
+                _unsubscribeOptions.UserProperties = new List<MqttUserProperty>();
+            }
+
+            _unsubscribeOptions.UserProperties.Add(userProperty);
+
+            return this;
+        }
+
+        public MqttClientUnsubscribeOptionsBuilder WithTopicFilter(string topic)
+        {
+            if (topic is null) throw new ArgumentNullException(nameof(topic));
+
+            if (_unsubscribeOptions.TopicFilters is null)
+            {
+                _unsubscribeOptions.TopicFilters = new List<string>();
+            }
+
+            _unsubscribeOptions.TopicFilters.Add(topic);
+
+            return this;
+        }
+        
+        public MqttClientUnsubscribeOptionsBuilder WithTopicFilter(TopicFilter topicFilter)
+        {
+            if (topicFilter is null) throw new ArgumentNullException(nameof(topicFilter));
+
+            return WithTopic(topicFilter.Topic);
+        }
+
+        public MqttClientUnsubscribeOptions Build()
+        {
+            return _unsubscribeOptions;
+        }
+    }
+}