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/Models/ValueObjects/AnnotationOrigin.cs

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+namespace DocumentService.Client.Models.ValueObjects;
+
+/// <summary>
+/// Coordinate origin point for PDF annotations.
+/// </summary>
+public enum AnnotationOrigin
+{
+    /// <summary>
+    /// Bottom-left corner (PDF native coordinate system, default)
+    /// </summary>
+    BottomLeft,
+    
+    /// <summary>
+    /// Top-left corner (common in UI frameworks)
+    /// </summary>
+    TopLeft
+}

DocumentService.Client/Models/ValueObjects/AnnotationType.cs

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+namespace DocumentService.Client.Models.ValueObjects;
+
+/// <summary>
+/// Supported PDF annotation types
+/// </summary>
+public enum AnnotationType
+{
+    /// <summary>
+    /// Text markup annotation (highlight, underline, strikeout)
+    /// </summary>
+    TextMarkup,
+    
+    /// <summary>
+    /// Free text annotation (text box with visible text)
+    /// </summary>
+    FreeText,
+    
+    /// <summary>
+    /// Sticky note annotation (popup comment icon)
+    /// </summary>
+    StickyNote,
+    
+    /// <summary>
+    /// Circle shape annotation
+    /// </summary>
+    Circle,
+    
+    /// <summary>
+    /// Square shape annotation
+    /// </summary>
+    Square
+}

DocumentService.Client/Models/ValueObjects/OnReconfigure.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DocumentService.Client.Models.ValueObjects;
+
+public enum OnReconfigure
+{
+    ThrowException = 0,
+    Ignore = 1,
+}

DocumentService.Client/Models/ValueObjects/PredefinedStampType.cs

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+namespace DocumentService.Client.Models.ValueObjects;
+
+/// <summary>
+/// Predefined stamp types with standard text and styling.
+/// </summary>
+public enum PredefinedStampType
+{
+    /// <summary>
+    /// CONFIDENTIAL stamp (red, bold).
+    /// </summary>
+    Confidential,
+
+    /// <summary>
+    /// APPROVED stamp (green, bold).
+    /// </summary>
+    Approved,
+
+    /// <summary>
+    /// DRAFT stamp (gray, italic).
+    /// </summary>
+    Draft,
+
+    /// <summary>
+    /// VOID stamp (red, strikethrough effect).
+    /// </summary>
+    Void,
+
+    /// <summary>
+    /// FOR REVIEW stamp (orange, bold).
+    /// </summary>
+    ForReview
+}

DocumentService.Client/Models/ValueObjects/StampPlacement.cs

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+namespace DocumentService.Client.Models.ValueObjects;
+
+/// <summary>
+/// Specifies whether the stamp should appear in the foreground or background.
+/// </summary>
+public enum StampPlacement
+{
+    /// <summary>
+    /// Stamp appears on top of existing page content.
+    /// </summary>
+    Foreground,
+
+    /// <summary>
+    /// Stamp appears behind existing page content (watermark effect).
+    /// </summary>
+    Background
+}

DocumentService.Client/Models/ValueObjects/StampType.cs

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+namespace DocumentService.Client.Models.ValueObjects;
+
+/// <summary>
+/// Specifies the type of stamp to add to a PDF document.
+/// </summary>
+public enum StampType
+{
+    /// <summary>
+    /// Text-based stamp with custom text, font, and color.
+    /// </summary>
+    Text,
+
+    /// <summary>
+    /// Image-based stamp (PNG/JPEG overlay).
+    /// </summary>
+    Image,
+
+    /// <summary>
+    /// Predefined stamp with standard text (e.g., CONFIDENTIAL, APPROVED).
+    /// </summary>
+    Predefined
+}

DocumentService.Client/Models/ValueObjects/TextMarkupStyle.cs

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+namespace DocumentService.Client.Models.ValueObjects;
+
+/// <summary>
+/// Text markup annotation style (highlight, underline, strikeout)
+/// </summary>
+public enum TextMarkupStyle
+{
+    /// <summary>
+    /// Highlight text with background color
+    /// </summary>
+    Highlight,
+    
+    /// <summary>
+    /// Underline text
+    /// </summary>
+    Underline,
+    
+    /// <summary>
+    /// Strikeout text (strikethrough)
+    /// </summary>
+    Strikeout
+}