Feature Request: Advanced Select optgroup

Hello, I'd like to request optgroup functionality for the advanced select field.

This would help a lot when managing large data sets.

It looks like it is possible within the select 2 library: https://select2.org/data-sources/formats#grouped-data

I know this was requested here: https://support.metabox.io/topic/callback-function-with-optgroup/

Thanks!

Jacob Hill