Merge branch 'bugfix/AttributeDescriptor.ModelType' into develop

Fixes a bug in which the `ModelType` could not be dynamically assigned based on the current attribute values, as is required for e.g. the `TopicListAttributeDescriptor` and the `TokenizedTopicListAttributeDescriptor` in the OnTopic Editor.

Author: JeremyCaney

OnTopic/Metadata/AttributeDescriptor.cs

@@ -90,7 +90,7 @@ public AttributeDescriptor(
     ///   reduces these down into a single type based on how they're exposed in the Topic Library, not based on how they're
     ///   exposed in the editor.
     /// </remarks>
-    public ModelType ModelType { get; protected init; } = ModelType.ScalarValue;
+    public virtual ModelType ModelType { get; protected init; } = ModelType.ScalarValue;
 
     /*==========================================================================================================================
     | PROPERTY: EDITOR TYPE