Then IMO you get this wrong, if you want to replace whole `PurchaseStep`. Point of this PR is to create easy extension point for replacement of payment library/manager, not about rewriting whole step.

I understand that this could be specific right now for Payum, but as I said, I didn't used any other library/manager with Sylius, so I __can't__ give the correct suggestion what data could be useful or not for other library here.