Add PDF signing feature with UI and backend support

Introduced a new `PdfSigningService` for digitally signing PDFs
using a PFX certificate and DevExpress's `PdfDocumentSigner`.
Updated `Program.cs` to register the service and added a new
configuration section `PdfSigning` in `appsettings.json` for
certificate management.

Added a Razor Page `TestSignature.cshtml` and its page model
`TestSignature.cshtml.cs` to provide a user interface for testing
PDF signing. The page includes file upload, signature validation,
and result display functionality.

Implemented supporting classes `PdfSigningOptions` for signature
customization and `SignatureInformation` for extracting and
displaying signature details, including signer name, location,
and certificate validity.

DXApp.TemplateKitProject/appsettings.json

@@ -17,5 +17,11 @@
   },
   "PdfResults": {
     "OutputDirectory": "C:\\PdfResults"
+  },
+  "PdfSigning": {
+    "CertificatePath": "C:\\PdfCertificates\\dxapp-test.pfx",
+    "CertificatePassword": "DXAppTest123!",
+    "SignerName": "DXApp Test System",
+    "Location": "Digital Data AG"
   }
 }