Allow user properties as pre-encoded UTF-8 binary buffer (#2228)

* Add support for binary user properties in MQTT 5.0

* Optimize byte array reversal in SubscriptionTopicHashTests

* Use InvariantCulture for topic string conversion in Load_Tests

* switched to `Microsoft.Testing.Platform`

* Refactor CI workflow to separate signed and unsigned package builds

* Add condition to sign job to prevent execution on external pull requests

* Remove fallback to value as string in MqttUserProperty

* Refactor MqttUserProperty handling to use byte arrays for improved performance and add deprecation warnings for legacy methods

Samples/Client/Client_Publish_Samples.cs

@@ -6,6 +6,9 @@
 // ReSharper disable UnusedMember.Global
 // ReSharper disable InconsistentNaming
 
+using System;
+using System.Text;
+
 namespace MQTTnet.Samples.Client;
 
 public static class Client_Publish_Samples
@@ -84,4 +87,21 @@ public static class Client_Publish_Samples
 
         Console.WriteLine("MQTT application message is published.");
     }
+
+    public static MqttApplicationMessage Create_Message_With_Binary_User_Property()
+    {
+        /*
+         * MQTT v5 user properties are encoded as UTF-8 strings. When the UTF-8 payload is already available
+         * as a byte buffer, the builder APIs can avoid creating intermediate strings.
+         */
+
+        var encodedValue = Encoding.UTF8.GetBytes("sensor-01");
+
+        return new MqttApplicationMessageBuilder()
+            .WithTopic("samples/metadata/binary")
+            .WithUserProperty("client-id", encodedValue.AsMemory())
+            .WithUserProperty("checksum", new ArraySegment<byte>(encodedValue))
+            .WithPayload("metadata")
+            .Build();
+    }
 }

Samples/Client/Client_Subscribe_Samples.cs

@@ -7,6 +7,7 @@
 // ReSharper disable InconsistentNaming
 // ReSharper disable UnusedMember.Local
 
+using System.Text;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Samples.Helpers;
@@ -67,7 +68,7 @@ public static class Client_Subscribe_Samples
             args.ResponseReasonString = "That did not work!";
 
             // User properties require MQTT v5!
-            args.ResponseUserProperties.Add(new MqttUserProperty("My", "Data"));
+            args.ResponseUserProperties.Add(new MqttUserProperty("My", Encoding.UTF8.GetBytes("Data")));
 
             // Now the broker will resend the message again.
             return Task.CompletedTask;