Add a way to model custom fields and layouts without adding them globally

In order to test potential improvements to our workspace custom layouts, we have to create a separate test workspace and then clone and update the custom layouts. If we want to do any changes to the custom fields (such as new values), we also have to copy those and add them to the new layout. I'd like to be able to model out changes to our existing layouts without having to do duplication and then rationalization of those changes after we've tested them.