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/MqttApplicationMessageValidator_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.Formatter;
 
 namespace MQTTnet.Tests;
@@ -27,7 +28,7 @@ public sealed class MqttApplicationMessageValidator_Tests
     public void Succeed_When_Using_UserProperties_And_MQTT_500()
     {
         MqttApplicationMessageValidator.ThrowIfNotSupported(
-            new MqttApplicationMessageBuilder().WithTopic("A").WithUserProperty("User", "Property").Build(),
+            new MqttApplicationMessageBuilder().WithTopic("A").WithUserProperty("User", Encoding.UTF8.GetBytes("Property")).Build(),
             MqttProtocolVersion.V500);
     }
 
@@ -35,7 +36,7 @@ public sealed class MqttApplicationMessageValidator_Tests
     public void Succeed_When_Using_WillUserProperties_And_MQTT_311()
     {
         Assert.ThrowsExactly<NotSupportedException>(() => MqttApplicationMessageValidator.ThrowIfNotSupported(
-            new MqttApplicationMessageBuilder().WithTopic("B").WithUserProperty("User", "Property").Build(),
+            new MqttApplicationMessageBuilder().WithTopic("B").WithUserProperty("User", Encoding.UTF8.GetBytes("Property")).Build(),
             MqttProtocolVersion.V311));
     }
 }