add CreatedAt for survey and created repository interface for answer variants

SurveyLib.Core/Models/Survey.cs

@@ -7,6 +7,7 @@ public class Survey
     public string Description { get; set; }
 
     public int? CreatedBy { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 
     public ICollection<QuestionBase> Questions { get; set; }
     public ICollection<Completion> Completions { get; set; }