Refactor data layer, add business layer

BusinessLayer/Services/IService.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+
+namespace BusinessLayer.Services
+{
+    public interface IService<T>
+        where T : class
+    {
+        IEnumerable<T> GetAll(int? limit, int? offset);
+        T? GetById(Guid id);
+        void Add(T entity);
+        void Update(T entity);
+        void Delete(Guid id);
+    }
+}