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.
