Add Identity configuration, DTOs and Mappers

-Configured Identity using a custom User model to manage authentication and authorization, to be used in the project.
- Added multiple DTOs (Data Transfer Objects).
- Added multiple Mappers ( manual mapping).
- Updated AppDbContext and User model to fully integrate with Identity framework for user management.
-Added relationships between models.
Note : no migration yet

survey-beta/DTOs/Default/QuestionDto.cs

@@ -0,0 +1,10 @@
+namespace survey_beta.DTOs.Default
+{
+    public class QuestionDto
+    {
+        public string Id { get; set; }
+        public string Content { get; set; }
+        public string SurveyId { get; set; }
+        public ICollection<ChoiceDto> Choices { get; set; }
+    }
+}