add LdtkFieldInstance and LdtkEntityMap classes; update LdtkSceneBuilder to support entity mapping

addons/csharp_ldtk_importer/Models/LdtkFieldInstance.cs

@@ -0,0 +1,16 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace CSharpLdtkImporter.Models;
+
+public class LdtkFieldInstance
+{
+    [JsonPropertyName("__identifier")]
+    public string Identifier { get; set; }
+
+    [JsonPropertyName("__type")]
+    public string Type { get; set; }
+
+    [JsonPropertyName("__value")]
+    public JsonElement Value { get; set; }
+}