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/IPdfConversionClient.cs

@@ -0,0 +1,49 @@
+namespace DocumentService.Client.Interfaces;
+
+/// <summary>
+/// Client for PDF conversion operations (PDF ? PDF/A).
+/// </summary>
+/// <remarks>
+/// All methods on this interface are marked obsolete because the corresponding
+/// API endpoints are not yet implemented.
+/// </remarks>
+public interface IPdfConversionClient
+{
+    /// <summary>
+    /// Converts a standard PDF to PDF/A format (multipart).
+    /// </summary>
+    /// <param name="pdfStream">Source PDF stream</param>
+    /// <param name="pdfALevel">Target PDF/A level (e.g., "PDF/A-3b")</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>PDF/A document as stream</returns>
+    [Obsolete("API endpoint not implemented yet")]
+    Task<Stream> ConvertToPdfAAsync(Stream pdfStream, string pdfALevel = "PDF/A-3b", CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Converts a standard PDF to PDF/A format (Base64).
+    /// </summary>
+    /// <param name="pdfBytes">Source PDF as byte array</param>
+    /// <param name="pdfALevel">Target PDF/A level (e.g., "PDF/A-3b")</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>PDF/A document as stream</returns>
+    [Obsolete("API endpoint not implemented yet")]
+    Task<Stream> ConvertToPdfAAsync(byte[] pdfBytes, string pdfALevel = "PDF/A-3b", CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Converts a PDF/A document to standard PDF (multipart).
+    /// </summary>
+    /// <param name="pdfStream">Source PDF/A stream</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Standard PDF as stream</returns>
+    [Obsolete("API endpoint not implemented yet")]
+    Task<Stream> ConvertFromPdfAAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Converts a PDF/A document to standard PDF (Base64).
+    /// </summary>
+    /// <param name="pdfBytes">Source PDF/A as byte array</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Standard PDF as stream</returns>
+    [Obsolete("API endpoint not implemented yet")]
+    Task<Stream> ConvertFromPdfAAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+}