[FEATURE] Add rendering support for UI Toolkit

[mattstromandev]

Feature description

Currently the SubClassSelectorDrawer is only implemented using IMGUI and does not have a UI Toolkit implementation. This presents a limitation for recent versions of Unity where many people will be using UI Toolkit for rendering custom elements.

UI Toolkit implementations can render nested IMGUI ones, but once we hit IMGUI we can not go back to UI Toolkit as nested, so this is an example of what we end up with:

Adding the implementation for UI Toolkit will properly support both systems.