ReadonlyExpandablePropertyDrawer: initial commit.

[danineuss]

@dbrizov (or any of the maintainers) I hope you are in a place to take a look at this. Thank you for your all your hard work 💛

Custom drawer for any field that references a scriptable object much like ExpandablePropertyDrawer. Shows the contents of the scriptable object without making the properties thereof editable.

This is how it looks in the Inspector:

Note how the field of the config itself remains editable, meaning you can still switch out which config is used (in contrast to the ReadonlyAttribute where that is prevented). Only the properties of the inner config are read-only.