Improvements to dataloaders for dealing with inheritance

PkmnLib.Dataloader/Models/SerializedItem.cs

@@ -1,3 +1,7 @@
+using System.Collections.Generic;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
 namespace PkmnLib.Dataloader.Models;
 
 public class SerializedItem
@@ -8,4 +12,7 @@ public class SerializedItem
     public string[] Flags { get; set; } = null!;
     public int Price { get; set; }
     public byte FlingPower { get; set; }
+    
+    [JsonExtensionData]
+    public Dictionary<string, JsonElement>? ExtensionData { get; set; }
 }