1846 server mqttserverstopasync doesnt indicate correct reason (#1872)

Source/MQTTnet/Server/Stopping/MqttServerStopOptionsBuilder.cs

@@ -0,0 +1,39 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using MQTTnet.Server.Disconnecting;
+
+namespace MQTTnet.Server
+{
+    public sealed class MqttServerStopOptionsBuilder
+    {
+        readonly MqttServerStopOptions _options = new MqttServerStopOptions();
+
+        public MqttServerStopOptionsBuilder WithDefaultClientDisconnectOptions(MqttServerClientDisconnectOptions value)
+        {
+            _options.DefaultClientDisconnectOptions = value;
+            return this;
+        }
+        
+        public MqttServerStopOptionsBuilder WithDefaultClientDisconnectOptions(Action<MqttServerClientDisconnectOptionsBuilder> builder)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            var optionsBuilder = new MqttServerClientDisconnectOptionsBuilder();
+            builder.Invoke(optionsBuilder);
+            
+            _options.DefaultClientDisconnectOptions = optionsBuilder.Build();
+            return this;
+        }
+        
+        public MqttServerStopOptions Build()
+        {
+            return _options;
+        }
+    }
+}