Fix support for enhanced authentication (#2132)

* Implement enhanced authentication for client and server.

* Update samples

* Add abstraction layer

* Merge master

* Use dedicated class for public API.

* Update release notes

* Rename methods to fit naming scheme

* Fix unit tests and samples

Source/MQTTnet.Server/EnhancedAuthentication/ExchangeEnhancedAuthenticationOptionsFactory.cs

@@ -0,0 +1,70 @@
+// 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.Text;
+using MQTTnet.Packets;
+
+namespace MQTTnet.Server.EnhancedAuthentication;
+
+public sealed class ExchangeEnhancedAuthenticationOptionsFactory
+{
+    readonly ExchangeEnhancedAuthenticationOptions _options = new();
+
+    public ExchangeEnhancedAuthenticationOptions Build()
+    {
+        return _options;
+    }
+
+    public ExchangeEnhancedAuthenticationOptionsFactory WithAuthenticationData(byte[] authenticationData)
+    {
+        _options.AuthenticationData = authenticationData;
+
+        return this;
+    }
+
+    public ExchangeEnhancedAuthenticationOptionsFactory WithAuthenticationData(string authenticationData)
+    {
+        if (authenticationData == null)
+        {
+            _options.AuthenticationData = null;
+        }
+        else
+        {
+            _options.AuthenticationData = Encoding.UTF8.GetBytes(authenticationData);
+        }
+
+        return this;
+    }
+
+    public ExchangeEnhancedAuthenticationOptionsFactory WithReasonString(string reasonString)
+    {
+        _options.ReasonString = reasonString;
+
+        return this;
+    }
+
+    public ExchangeEnhancedAuthenticationOptionsFactory WithUserProperties(List<MqttUserProperty> userProperties)
+    {
+        _options.UserProperties = userProperties;
+
+        return this;
+    }
+
+    public ExchangeEnhancedAuthenticationOptionsFactory WithUserProperty(string name, string value)
+    {
+        if (name == null)
+        {
+            throw new ArgumentNullException(nameof(name));
+        }
+
+        if (_options.UserProperties == null)
+        {
+            _options.UserProperties = new List<MqttUserProperty>();
+        }
+
+        _options.UserProperties.Add(new MqttUserProperty(name, value));
+
+        return this;
+    }
+}