Implement invoice payment

This document describes the steps to consider implementing the invoice payment method without integrating with a third-party payment service provider.

If this is your first time creating a new module, see Tutorial: Add a new module.

1. Implement invoice payment in frontend.
2. Integrate invoice payment into checkout.
3. Implement invoice payment in backend.
4. Implement invoice payment in shared layer.
5. Test the invoice payment implementation.