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

Entities/DTO/Exam/AssigExamToStudentsDto.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+	public class AssigExamToStudentsDto
+	{
+		public Guid CreaterId { get; set; }
+		public Guid ExamId { get; set; }
+		public Guid ClassId { get; set; }
+		public List<Guid> StudentIds { get; set; } = new List<Guid>();
+	}
+
+	public record AssigExamToClassDto(Guid examId, Guid classId);
+}