Explore changes required to support configs with Rails 6.2/7

[elrayle]

@jrochkind raised an issue in Slack that upcoming releases of Rails may have a problem with the way engines define configs.

rails issue rails/rails#40904 is a good one for engine authors to be aware of though, it sounds like some things are going to break in rails 7 if not 6.2. If you have a class you use for config in app/ or another auto-loaded directory, it is deprecated and will break soon enough.

[jrochkind]

An attempt to fix this at #349 was reverted at #350.