Add nullable Annotations property to SignatureDto

A new property `Annotations` of type `IEnumerable<AnnotationDto>?`
was added to the `SignatureDto` class. This property includes a
getter and setter, allowing the class to manage a collection of
annotations. The property is nullable, enabling it to hold a
`null` value if no annotations are present.

EnvelopeGenerator.Application/Common/Dto/SignatureDto.cs

@@ -93,4 +93,9 @@ public class SignatureDto : ISignature
     /// Gets or sets the left position of the element (in layout terms).
     /// </summary>
     public double Left => X;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public IEnumerable<AnnotationDto>? Annotations { get; set; }
 }