refactor: standardize annotation types and clean up PDF burn handler

- Introduced `AnnotationType.PSPDFKit` constants for consistent annotation type references.
- Updated `BurnPdfCommandHandler` to use new PSPDFKit annotation types.
- Refactored method signatures and local variables for clarity (e.g., using 'var', null-coalescing for annotation properties).
- Streamlined `using` statements and memory management in PDF burning process.
- Added placeholder methods for form fields, image, and ink annotations to centralize future implementations.

EnvelopeGenerator.Domain/Constants/AnnotationType.cs

@@ -0,0 +1,13 @@
+namespace EnvelopeGenerator.Domain.Constants
+{
+    public static class AnnotationType
+    {
+        public static class PSPDFKit
+        {
+            public const string Image = "pspdfkit/image";
+            public const string Ink = "pspdfkit/ink";
+            public const string Widget = "pspdfkit/widget";
+            public const string FormField = "pspdfkit/form-field-value";
+        }
+    }
+}