feat StayExport - Refactor Export Services.

- Created new service: IStayExportService" for common export operations.
- Updated older services to be "IStayExportFormatProvider". This services will only implement the export format.
- Renamed "IStayExportServiceFactory" to "IStayExportFormatProviderFactory" as this factory now is used to get the format providers for the given format.

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

@@ -0,0 +1,27 @@
+using System.Text.Json;
+using Somnia.GuestLog.Enums.Models;
+using Somnia.GuestLog.Models.DTOs;
+
+namespace Somnia.GuestLog.Services.StayExport;
+
+public class StayExportProviderJson : IStayExportFormatProvider
+{
+    private const string fileExtension = "json";
+
+    public StayExportFormat GetFormat() => StayExportFormat.GuestLogJson;
+    public string GetFilename(DateTime exportTime)
+        => $"{Enum.GetName(GetFormat())}_{exportTime.ToFileTimeUtc()}.{fileExtension}";
+
+
+    public async Task<StayExportFileResult> ExportDataAsync(IEnumerable<StayEntryDTO> stayList, Stream dataStream)
+    {
+        if (!stayList.Any())
+        {
+            return StayExportFileResult.NoData;
+        }
+
+        // Convert to JSON.
+        await JsonSerializer.SerializeAsync(dataStream, stayList);
+        return StayExportFileResult.OK;
+    }
+}