Add ZUGFeRD detection and extraction functionality

Introduced `ZugferdController` to handle ZUGFeRD-related operations, including detection and extraction of ZUGFeRD XML from PDFs via file upload or Base64-encoded payloads. Integrated `MediatR` for query/command handling.

Added `ZugferdSettings` for configurable file names and patterns, and updated `appsettings.json` and `Program.cs` to support this configuration.

Implemented `HasZugferdQuery` and `ExtractZugferdCommand` with their respective handlers and validators. Added DTOs (`ZugferdCheckResult`, `ZugferdExtractionResult`) for operation results.

Included `ZUGFeRD-Example.pdf` for testing and integrated `Serilog.Ui.Core.Extensions` for logging.

DocumentOperator.API/appsettings.json

@@ -61,6 +61,22 @@
     "EnableDetailedLogging": true
   },
 
+  "ZugferdSettings": {
+    "ZugferdFileNames": [
+      "factur-x.xml",
+      "zugferd-invoice.xml",
+      "ZUGFeRD-invoice.xml",
+      "xrechnung.xml",
+      "XRechnung.xml"
+    ],
+    "ZugferdFileNamePatterns": [
+      "factur",
+      "zugferd",
+      "xrechnung",
+      "peppol"
+    ]
+  },
+
   "RedisSettings": {
     "ConnectionString": "localhost:6379",
     "InstanceName": "DocumentOperator:",