[8.x] Allow to store live and sandbox credentials for easy switching

Ok, this is it! Done!

Wouldn't it be better to clearly document how to override the credentials via settings.php?

@damienmckenna thanks for your reply. In my eyes both is true.

Currently the issue is, that theres a "Live" vs. "Sandbox" switch, but you can only store public / private key for one of them. This combination doesn't make much sense in our eyes.
When switching the radio, you have to exchange the keys and enter the opposite ones and when switching back, same again...

The goal here is to add separate fields & config values for live and sandbox credentials, so you can switch at any time, if you need to, by just switching the radio.

Of course it should also be documented how to set these values in settings.php.

So let's wait for final feedback how to finish this then, or any additions @damienmckenna?

FWIW just some encouragement to go head with this: Overriding is certainly an alternative, but far less discoverable. The UI is already there and other payment modules use it correctly, so this is currently quite counter-intuitive. It should therefore be implemented IMHO. It's useful!