Allow to switch between live and sandbox in settings form

Problem/Motivation

Like in many other e.g. payment modules, you typically have a live and a sandbox environment at Billwerk you're playing with.

While it should be the admin's task to ensure both are "in sync" and switching doesn't break things, we might add a setting to switch between sandbox and live crecentials (if configured).

I guess this should be combined with ✨ Add a status report entry and logic to ensure the billwerk API credentials are present Active and is surely a nice to have convenience feature.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes