Add PsPdfKitAnnotation record with deprecation notice

Introduced a new `PsPdfKitAnnotation` record in the namespace
`EnvelopeGenerator.Application.Common.Dto` to represent PSPDFKit
annotation data. The record includes two parameters: `Instant`
of type `ExpandoObject` and `Structured` of type
`IEnumerable<AnnotationCreateDto>`.

Marked the record with the `[Obsolete]` attribute to indicate
that the PSPDFKit library is deprecated. Added a summary XML
comment to describe the purpose of the record. Also added a
`using` directive for `System.Dynamic` to support `ExpandoObject`.

EnvelopeGenerator.Application/Common/Dto/PsPdfKitAnnotation.cs

@@ -0,0 +1,11 @@
+using System.Dynamic;
+
+namespace EnvelopeGenerator.Application.Common.Dto;
+
+/// <summary>
+/// Represents PSPDFKit annotation data.
+/// </summary>
+/// <param name="Instant">Instant annotation data.</param>
+/// <param name="Structured">Structured annotation data.</param>
+[Obsolete("The PSPDFKit library is deprecated.")]
+public record PsPdfKitAnnotation(ExpandoObject Instant, IEnumerable<AnnotationCreateDto> Structured);