Add object field

[shash7]

Expected behavior

Add a Object field which shows arbitrary object or array data in a textarea-esque field. You should be able to edit the field. Formatting would be optional.

Actual/Current behavior

There is no such field at the moment. I have some custom document paths I would like to display in the admin area. Since they are quite random, it would be good to make a simple object field to show them and make them editable.

Steps to reproduce the actual/current behavior

Ideally, merge #4855 as it seems like similar to what I'm looking for.
Also related to #2488

[laurenskling]

Keystone 4 is going in maintenance mode, see #4913. No major changes are expected.