Refactored DocStatusHandler to assign a blank JSON object ("{}")
to the Value property when PsPdfKitAnnotation is not present or
not of the expected type, preventing potential runtime errors.
- 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`
