Implement/test the other payment options

I did some testing, and on the testing endpoint all methods seem to work. In this patch I made the implementation to add the option to the payment-provider settings to be able to select which options can be chosen.

Next is to add a select to the checkout screen. I have done some digging, but this is not a simple task. Maybe a form alter.

After a lot of fiddling around, I managed to complete this task. The behaviour is as follows:
- You can configurate the available payment methods in the provider config
- When only one payment method is available checkout redirects to the payment provider directly
- When the option CARDS is chosen checkout redirects to the payment provider directly (card type is chosen there)
- When the option EXTERNAL is chosen checkout redirects to the payment provider directly (payment is chosen there)
- When multiple options (but not EXTERNAL) ar available an extra checkout step is available in which the payment method is chosen

Committed this patch to the module.