Add new PowerShell project

Source/MQTTnet.PowerShell/Cmdlets/UnsubscribeMqttTopicCmdlet.cs

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+using System.Management.Automation;
+
+namespace MQTTnet.PowerShell.Cmdlets;
+
+[Cmdlet("Unsubscribe", "MqttTopic")]
+[OutputType(typeof(MqttClientUnsubscribeResult))]
+public class UnsubscribeMqttTopicCmdlet : PSCmdlet
+{
+    [Parameter(Mandatory = true, ValueFromPipeline = true)]
+    public required MqttSession Session { get; set; }
+
+    [Parameter(Mandatory = true)]
+    public required string Topic { get; set; }
+
+    protected override void ProcessRecord()
+    {
+        var response = Session.Client.UnsubscribeAsync(Topic).GetAwaiter().GetResult();
+
+        WriteObject(response);
+    }
+}