Add PKCS#11 support for PDF signing and validation

Introduced `Pkcs11Options` for configuring PKCS#11 signing workflows, including properties for library path, token selection, and signing metadata. Added `PdfSignOptionsDto` to represent signing options and `PdfSignatureCheckResultDto` with related DTOs for signature validation.

Defined `PdfSignatureCertificationLevel` enum for certification levels. Added `IPdfDigitalSignatureService` interface to support signing and validating PDF signatures. These changes enable robust and extensible handling of digital signatures.

EnvelopeGenerator.Application/Common/Interfaces/Services/IPdfDigitalSignatureService.cs

@@ -0,0 +1,38 @@
+using EnvelopeGenerator.Application.Common.Dto;
+
+namespace EnvelopeGenerator.Application.Common.Interfaces.Services;
+
+/// <summary>
+/// Defines PDF digital-signing operations for the application boundary.
+/// </summary>
+public interface IPdfDigitalSignatureService
+{
+    /// <summary>
+    /// Creates a digital signature for the provided PDF bytes and returns
+    /// a new signed PDF document.
+    /// </summary>
+    /// <param name="pdfBytes">Source PDF content as a byte array.</param>
+    /// <param name="options">Optional signing options that map to PDF signature metadata fields.</param>
+    /// <param name="cancellationToken">Cancellation token for cooperative cancellation.</param>
+    /// <returns>The signed PDF as a byte array.</returns>
+    Task<byte[]> SignPdfAsync(byte[] pdfBytes, PdfSignOptionsDto? options = null, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Applies multiple signatures to the provided PDF in sequence and returns
+    /// the final signed PDF document.
+    /// </summary>
+    /// <param name="pdfBytes">Source PDF content as a byte array.</param>
+    /// <param name="optionsList">Ordered signing options; one signature is applied per item.</param>
+    /// <param name="cancellationToken">Cancellation token for cooperative cancellation.</param>
+    /// <returns>The signed PDF as a byte array.</returns>
+    Task<byte[]> SignPdfAsync(byte[] pdfBytes, IReadOnlyList<PdfSignOptionsDto> optionsList, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Checks whether the provided PDF contains signature field(s) and whether
+    /// those signatures are cryptographically valid.
+    /// </summary>
+    /// <param name="pdfBytes">PDF content to inspect.</param>
+    /// <param name="cancellationToken">Cancellation token for cooperative cancellation.</param>
+    /// <returns>A structured result containing signature presence and per-signature details.</returns>
+    Task<PdfSignatureCheckResultDto> CheckPdfSignatureAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+}