refactor: modularize report designer components and controllers into a new report-designer module.

src/backend/Zentral.API/Controllers/VirtualDatasetsController.cs

@@ -1,4 +1,5 @@
 using System.Text.Json;
+using Zentral.API.Services.Reports;
 using Zentral.Domain.Entities;
 using Zentral.Infrastructure.Data;
 using Microsoft.AspNetCore.Mvc;