Added option for how many subscriptions to send in a single call to t… (#1209)

* Added option for how many subscriptions to send in a single call to the server.  aws iotcore limits this to 8

* Split subscription messages up to 8 at a time when reconnecting

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClientOptionsBuilder.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using MQTTnet.Client.Options;
 using MQTTnet.Server;
 
@@ -68,7 +68,13 @@ namespace MQTTnet.Extensions.ManagedClient
             options(_clientOptionsBuilder);
             return this;
         }
-        
+
+        public ManagedMqttClientOptionsBuilder WithMaxSubcribeUnsubscribeMessagesAtOnce(int? value)
+        {
+            _options.MaxSubcribeUnsubscribeMessagesAtOnce = value;
+            return this;
+        }
+
         public ManagedMqttClientOptions Build()
         {
             if (_clientOptionsBuilder != null)