Renamed namespaces and projects from DocumentOperator to DocumentService, updating all references accordingly. Refactored PdfSignatureAppearanceDto and related enums to use properties with init accessors for improved clarity. Removed and re-added IPkcs11SelfTestStatusProvider with updated namespace. Simplified dependency injection by removing DocumentOperatorApplication registration. Updated appsettings.json with a specific AssetsRootPath. Incremented version numbers in project files for a new release. Introduced PdfSignatureClient and IPdfSignatureClient for PDF digital-signing operations. Updated project files to use newer Microsoft.Extensions package versions. Removed all DocumentOperator references, completing the transition to DocumentService. Added new DTOs and enums for PDF signature options and appearance.
Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.
