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/ApiKeySettings.cs

@@ -0,0 +1,9 @@
+namespace DocumentOperator.Infrastructure.Configuration;
+
+public class ApiKeySettings
+{
+    public const string SectionName = "ApiKeySettings";
+
+    public bool EnableValidation { get; set; } = true;
+    public Dictionary<string, TenantInfo> Keys { get; set; } = new();
+}