Support for using GlobalID with configurable columns

[tylerwillingham]

I currently work in an application that's using auto-incremented primary keys for GIDs and would like to migrate to using our external_id columns that we've started adding to various models. We can sidestep rails/globalid and do this if we need to, but I sensed in #135 that there may be an appetite from others for this kind of thing.

I think this is achievable/configurable on a per-model basis pretty cleanly so I'll put up a PR with some passing tests and get a sense for what the maintainers think

[Kamilove137]

Nkann

[Kamilove137]

Hi