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/RedisSettings.cs

@@ -0,0 +1,10 @@
+namespace DocumentOperator.Infrastructure.Configuration;
+
+public class RedisSettings
+{
+    public const string SectionName = "RedisSettings";
+
+    public string ConnectionString { get; set; } = "localhost:6379";
+    public string InstanceName { get; set; } = "DocumentOperator:";
+    public int CacheExpirationMinutes { get; set; } = 60;
+}