Add interfaces and DTOs for PDF operations support

Introduced multiple interfaces and DTOs to support a wide range of
PDF-related operations, including attachment handling, conversion,
validation, annotation, stamping, and metadata extraction.

Key changes:
- Added `DocumentServiceClientOptions` for HTTP client config.
- Introduced `IPdfAttachmentClient` for attachment operations.
- Added `IPdfConversionClient` for PDF/A conversion (marked obsolete).
- Added `IPdfOperationsClient` for merge, annotate, and stamp ops.
- Introduced `IPdfValidationClient` for PDF and PDF/A validation.
- Added `ISwissQrCodeClient` for Swiss QR Code extraction.
- Added `IZugferdClient` for ZUGFeRD detection and extraction.
- Created request DTOs for Base64-encoded operations.
- Added enums for annotation, stamp, and validation configurations.

These changes provide a flexible and extensible foundation for
interacting with PDF documents, supporting both multipart and
Base64-encoded inputs, and ensuring type safety with enums and records.

DocumentService.Client/Interfaces/ISwissQrCodeClient.cs

@@ -0,0 +1,36 @@
+namespace DocumentService.Client.Interfaces;
+
+/// <summary>
+/// Client for Swiss QR Code extraction operations.
+/// </summary>
+public interface ISwissQrCodeClient
+{
+    /// <summary>
+    /// Extracts Swiss QR Code from PDF (multipart).
+    /// </summary>
+    /// <param name="pdfStream">PDF file stream</param>
+    /// <param name="raw">If true, returns raw QR text lines instead of parsed Bill object</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Swiss QR Code extraction result</returns>
+    Task<SwissQrCodeExtractionResult> ExtractSwissQrCodeAsync(Stream pdfStream, bool raw = false, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Extracts Swiss QR Code from PDF (Base64).
+    /// </summary>
+    /// <param name="pdfBytes">PDF file as byte array</param>
+    /// <param name="raw">If true, returns raw QR text lines instead of parsed Bill object</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Swiss QR Code extraction result</returns>
+    Task<SwissQrCodeExtractionResult> ExtractSwissQrCodeAsync(byte[] pdfBytes, bool raw = false, CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// DTO for Swiss QR Code extraction result
+/// </summary>
+public record SwissQrCodeExtractionResult
+{
+    /// <summary>Parsed Swiss QR bill object. <c>null</c> when <c>raw=true</c> was requested.</summary>
+    public object? Bill { get; init; }
+    /// <summary>Raw QR code text lines. Populated when <c>raw=true</c> was requested.</summary>
+    public List<string> RawLines { get; init; } = new();
+}