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/Configuration/DocumentServiceClientOptions.cs

@@ -0,0 +1,29 @@
+namespace DocumentService.Client.Configuration;
+
+/// <summary>
+/// Configuration options for DocumentService HTTP client.
+/// </summary>
+public class DocumentServiceClientOptions
+{
+    /// <summary>
+    /// Base URL of the DocumentService API.
+    /// </summary>
+    /// <example>https://api.example.com</example>
+    public string BaseUrl { get; set; } = "http://localhost:5000";
+
+    /// <summary>
+    /// HTTP request timeout duration.
+    /// </summary>
+    public TimeSpan Timeout { get; set; } = TimeSpan.FromMinutes(5);
+
+    /// <summary>
+    /// Maximum number of retry attempts for failed requests.
+    /// </summary>
+    public int MaxRetries { get; set; } = 3;
+
+    /// <summary>
+    /// Whether to throw exceptions on HTTP error responses (4xx, 5xx).
+    /// If false, returns null/default values instead.
+    /// </summary>
+    public bool ThrowOnError { get; set; } = true;
+}