Add PSPDFKit annotation model and utilities

Introduce classes and interfaces for modeling PDF annotations, including support for layout, relative positioning, background rendering, and color. Added Annotation, AnnotationParams, Background, Color, Extensions, and IAnnotation to EnvelopeGenerator.GeneratorAPI.Models.PsPdfKitAnnotation. Enables flexible annotation management and rendering.

EnvelopeGenerator.GeneratorAPI/Models/PsPdfKitAnnotation/IAnnotation.cs

@@ -0,0 +1,22 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models.PsPdfKitAnnotation;
+
+public interface IAnnotation
+{
+    string Name { get; }
+
+    double? Width { get; }
+
+    double? Height { get; }
+
+    double Left { get; }
+
+    double Top { get; }
+
+    Color? BackgroundColor { get; }
+
+    Color? BorderColor { get; }
+
+    string? BorderStyle { get; }
+
+    int? BorderWidth { get; }
+}