add ReturnsAttribute to make annotating API more convenient

csharp/app/Backend/Controllers/ReturnsAttribute.cs

@@ -0,0 +1,22 @@
+using System.Net;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Innovenergy.Backend.Controllers;
+
+public class ReturnsAttribute : ProducesResponseTypeAttribute
+{
+    public ReturnsAttribute(HttpStatusCode statusCode) : base((Int32)statusCode)
+    {
+    }
+}
+
+public class ReturnsAttribute<T> : ProducesResponseTypeAttribute
+{
+    public ReturnsAttribute(HttpStatusCode statusCode) : base(typeof(T), (Int32)statusCode)
+    {
+    }
+    
+    public ReturnsAttribute() : base(typeof(T), (Int32)HttpStatusCode.OK)
+    {
+    }
+}