Be able to show a reference type (eg asset, entity) as 'needed' and give/show a warning if it hasn't been set

[yaustar]

We would like to be able to set an attribute that is a reference type as 'needed' and show a warning in the inspector and on the scene hierarchy view (like a red !) to show that attention is needed.

Odin in Unity does something similar (but for the whole project): https://odininspector.com/tutorials/odin-project-validator/getting-started-with-odin-project-validator