Integrate Serilog and add configuration classes

Enhanced logging with Serilog, including request logging and
structured exception handling during startup. Added support
for the Options Pattern with new configuration classes:
`DocumentOperatorSettings`, `RedisSettings`, and
`ApiKeySettings`. Introduced `TenantInfo` class for tenant
management. Updated project files to include new dependencies
and removed unused `Configuration` folder reference.

DocumentOperator.Infrastructure/Configuration/DocumentOperatorSettings.cs

@@ -0,0 +1,11 @@
+namespace DocumentOperator.Infrastructure.Configuration;
+
+public class DocumentOperatorSettings
+{
+    public const string SectionName = "DocumentOperatorSettings";
+
+    public string TempFolderPath { get; set; } = string.Empty;
+    public int TempFileRetentionHours { get; set; }
+    public int MaxPdfSizeMB { get; set; }
+    public bool EnableDetailedLogging { get; set; }
+}