Add server implementation

MQTTnet.Core/Packets/MqttSubscribePacket.cs

@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace MQTTnet.Core.Packets
+{
+    public class MqttSubscribePacket : MqttBasePacket, IPacketWithIdentifier
+    {
+        public ushort PacketIdentifier { get; set; }
+        
+        public IList<TopicFilter> TopicFilters { get; set; } = new List<TopicFilter>();
+
+        public override string ToString()
+        {
+            var topicFiltersText = string.Join(",", TopicFilters.Select(f => $"{f.Topic}@{f.QualityOfServiceLevel}"));
+            return
+                $"{nameof(MqttSubscribePacket)} [PacketIdentifier={PacketIdentifier}] [TopicFilters={topicFiltersText}]";
+        }
+    }
+}