feat(DocSignedNotification): replace Annotations with PsPdfKitAnnotation in DocSignedNotification

- Introduced new record `PsPdfKitAnnotation` to encapsulate both Instant and Structured annotation data
- Updated `DocSignedNotification` to use `PsPdfKitAnnotation` instead of `ExpandoObject Annotations`
- Modified extension methods to accept and map `PsPdfKitAnnotation`
- Added reference to `EnvelopeGenerator.Application.Annotations.Commands` for `CreateAnnotationCommand`

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

@@ -33,6 +33,6 @@ public class AnnotationHandler : INotificationHandler<DocSignedNotification>
     {
         Envelope = new() { Id = notification.EnvelopeId },
         Receiver = new() { Id = notification.ReceiverId },
-        PSPDFKitInstant = notification.Annotations
+        PSPDFKitInstant = notification.PsPdfKitAnnotation.Instant
     }, cancel);    
 }