feat(AnnotationHandler): add handler for DocSignedNotification to create annotations

- Implemented AnnotationHandler to process DocSignedNotification events
- Sends CreateAnnotationCommand via MediatR with envelope, receiver, and annotations data

EnvelopeGenerator.Application/Common/Notifications/DocSigned/Handlers/AnnotationHandler.cs

@@ -0,0 +1,38 @@
+using EnvelopeGenerator.Application.Annotations.Commands;
+using MediatR;
+
+namespace EnvelopeGenerator.Application.Common.Notifications.DocSigned.Handlers;
+
+/// <summary>
+/// 
+/// </summary>
+public class AnnotationHandler : INotificationHandler<DocSignedNotification>
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    private readonly ISender _sender;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="sender"></param>
+    public AnnotationHandler(ISender sender)
+    {
+        _sender = sender;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="notification"></param>
+    /// <param name="cancel"></param>
+    /// <returns></returns>
+    /// <exception cref="NotImplementedException"></exception>
+    public Task Handle(DocSignedNotification notification, CancellationToken cancel) => _sender.Send(new CreateAnnotationCommand()
+    {
+        Envelope = new() { Id = notification.EnvelopeId },
+        Receiver = new() { Id = notification.ReceiverId },
+        PSPDFKitInstant = notification.Annotations
+    }, cancel);    
+}