Add starter code for chapter 8

No modifications for Complete folder yet.

Chapter08/Start/MyMediaCollection/Model/Medium.cs

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+using Dapper.Contrib.Extensions;
+using MyMediaCollection.Enums;
+
+namespace MyMediaCollection.Model
+{
+    public class Medium
+    {
+        [Key]
+        public int Id { get; set;  }
+        public string Name { get; set; }
+        public ItemType MediaType { get; set; }
+    }
+}