重构项目结构，移除Assignment相关功能，优化Submission模块

Entities/DTO/Exam/AssignmentClassDto.cs

@@ -0,0 +1,23 @@
+using Entities.Contracts;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Xml.Serialization;
+
+namespace Entities.DTO
+{
+
+
+	public class AssignmentClassDto
+	{
+		public ExamDto Assignment { get; set; }
+		public ClassDto ClassId { get; set; }
+		public DateTime AssignedAt { get; set; }
+	}
+
+
+
+
+}