feat StayExport - Initial Implementation

- Created models for DB and DTOs: StayExportFile
- Added required db migrations.
- Added StayExportService and StayExportServiceFactory for stay exporting services.
- Added configuration options required for StayExport.

src/Somnia.GuestLog/Services/StayExport/IStayExportServiceFactory.cs

@@ -0,0 +1,11 @@
+using Somnia.GuestLog.Enums.Models;
+
+namespace Somnia.GuestLog.Services.StayExport;
+
+public interface IStayExportServiceFactory
+{
+    /// <summary>
+    /// Returns the StayExport service implementation given current configuration.
+    /// </summary>
+    public IStayExportService GetService(StayExportFormat exportFormat);
+}