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

Source/MQTTnet.Tests/Server/Unsubscribe_Tests.cs

@@ -2,6 +2,7 @@
 // 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.Exceptions;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
@@ -45,7 +46,7 @@ public sealed class Unsubscribe_Tests : BaseTestClass
 
         var client = await testEnvironment.ConnectClient();
 
-        var unsubscribeOptions = testEnvironment.ClientFactory.CreateUnsubscribeOptionsBuilder().WithTopicFilter("X").WithUserProperty("A", "1").Build();
+        var unsubscribeOptions = testEnvironment.ClientFactory.CreateUnsubscribeOptionsBuilder().WithTopicFilter("X").WithUserProperty("A", Encoding.UTF8.GetBytes("1")).Build();
         await client.UnsubscribeAsync(unsubscribeOptions);
 
         CollectionAssert.AreEqual(unsubscribeOptions.UserProperties.ToList(), eventArgs.UserProperties);