Add a callback plugin

Been using this for a while know. Works good for us.

Creators may not set their own code RTBC. Needs review. Any further plans here @attisan?

In general this totally makes sense to me, I'm not just sure about possible security implications.
Perhaps this should better be a submodule to opt-into or use further permissions?

@attisan which benefits do you see compared to implementing the whole extra field in code?
https://www.webomelette.com/creating-pseudo-fields-drupal-8