Support German/Austrian version of sixpay/saferpay

Sixpay/saferpay is now 'worldline'. It might be a good idea to rename the plugin and change the API URL.

For the german and austrain market wordline is not available, but instead https://www.payone.com (cooperation Worldline and DSV-Gruppe (Deutscher Sparkassenverlag)). The Sixpay/saferpay plugin could be cloned, the endpoint behavior can be checked and if necessary adapted (https://explorer.payone.com/?endpoint=CreateHostedCheckout&scenario=Basic).

Start by examining the existing Sixpay/Saferpay plugin and the Payone CreateHostedCheckout endpoint documentation linked in the issue. Verify the endpoint behavior for the German and Austrian market, then determine whether the cloned integration needs adaptation; done means the Payone-based plugin supports the relevant checkout flow.

Written by the indexing model from the issue text.