Refactor PDF signing logic and appearance handling

Refactored `Pkcs11Options` by removing `DefaultCertificationLevel` and improving property documentation. Replaced `VisualSignature` in `PdfSignOptionsDto` with `Appearance` using the new `PdfSignatureAppearanceDto` class, which encapsulates signature appearance details, including font and color settings.

Enhanced `PdfSignatureCertificationLevel` enum with more descriptive values and added XML documentation. Updated `IPdfDigitalSignatureService` to support optional signature images and removed redundant methods.

Refactored `PdfDigitalSignatureService` to streamline appearance handling, validate inputs, and improve maintainability. Removed unused methods and added support for `PdfSignatureAppearanceDto`.

Simplified `PdfSignatureController` by consolidating form properties into `PdfSignOptionsDto`. Improved code readability and flexibility while removing redundant logic.

EnvelopeGenerator.Infrastructure.Crypt/Helper/PdfCertificationLevelExtensions.cs

@@ -0,0 +1,16 @@
+using DevExpress.Pdf;
+using EnvelopeGenerator.Application.Common.Enums;
+
+namespace EnvelopeGenerator.Infrastructure.Crypt.Helper;
+
+public static class PdfCertificationLevelExtensions
+{
+    public static PdfCertificationLevel ToDevExpress(this PdfSignatureCertificationLevel value) => value switch
+    {
+        PdfSignatureCertificationLevel.NoChangesAllowed => PdfCertificationLevel.NoChangesAllowed,
+        PdfSignatureCertificationLevel.FillForms => PdfCertificationLevel.FillForms,
+        PdfSignatureCertificationLevel.NoCertification => PdfCertificationLevel.NoCertification,
+        PdfSignatureCertificationLevel.FillFormsAndAnnotate => PdfCertificationLevel.FillFormsAndAnnotate,
+        _ => throw new ArgumentOutOfRangeException(nameof(value), value, null),
+    };
+}