Is it possible to use CredentialsManager::AppfileConfig for OTHER files besides Appfile?

Fastlane is pushing to have all configuration in various *files, like Appfile, Matchfile, Deliverfile, etc.

However, only the values from Appfile can be retrieved using `CredentialsManager::AppfileConfig.try_fetch_value()`.
This presents a problem as often we need to retrieve a value from configuration before running the action.

And without having ability to fetch values from other files, we are left with an inconsistency problem, where some configs are in Appfile, and others are scattered in ENV vars.