Add support for visual PDF signatures

Enhanced `PdfSignOptionsDto` with a `VisualSignature` property and introduced the `PdfVisualSignatureOptionsDto` class for configuring visual signature options. Updated `PdfDigitalSignatureService` to handle visual signatures, including page number resolution, signature bounds calculation, and appearance customization.

Refactored `PdfSignatureController` to support visual signatures by extending `SignPdfForm` with `SignatureImageFile` and `VSignature` properties. Improved validation for page numbers and signature dimensions. Streamlined `Sign` and `IsSigned` endpoints for better clarity and reduced redundancy.

Added XML documentation for improved API clarity and performed general code cleanup to enhance maintainability.

EnvelopeGenerator.Application/Common/Dto/PdfSignOptionsDto.cs

@@ -12,4 +12,5 @@ public sealed record PdfSignOptionsDto
     public string? Phone { get; set; }
     public string? ApplicationName { get; set; }
     public PdfSignatureCertificationLevel? CertificationLevel { get; set; }
+    public PdfVisualSignatureOptionsDto? VisualSignature { get; set; }
 }

EnvelopeGenerator.Application/Common/Dto/PdfVisualSignatureOptionsDto.cs

@@ -0,0 +1,10 @@
+namespace EnvelopeGenerator.Application.Common.Dto;
+
+public sealed record PdfVisualSignatureOptionsDto(
+    byte[] ImageBytes,
+    int PageNumber,
+    double Left,
+    double Bottom,
+    double Width,
+    double Height,
+    bool ShowLabels = false);