Allow admin.readOnly to take in a function

[paulpopus]

I think it would be very useful to allow the admin.readOnly field config take a function with the doc data being passed into it like the hooks, so that we can more easily change if the field is editable dynamically.

Example use case:
Slug field with a checkbox if it should be editable, if not checked, then the field remains readOnly and the value of the field would be generated, otherwise the editor can easily toggle it on.