Use Craft 3.1's environmental settings feature

[benface]

It seems like all the config settings of this plugin would be good candidates for Craft 3.1's environmental settings feature: https://docs.craftcms.com/v3/extend/environmental-settings.html

That would make it possible to use environment variables for them directly within the CP. It's possible right now by overriding the saved settings with a config/stripe-checkout.php file, but that's causing issues with project config, where saving the settings form in the CP writes the secret key in project.yaml and in the database.