[Ecommerce] [Backoffice] Don't use hardcoded PaymentProviders

[ultramegatom]

The Payment section of the Backoffice should be refactored to not use hardcoded values. Makes it impossible to use it along with custom providers unless you override the view script...

We should also get rid of the magic method call: getProvider_%s_amount and provide some usefull interface:

https://github.com/pimcore/pimcore/blob/master/bundles/EcommerceFrameworkBundle/Resources/views/admin_order/detail.html.twig#L181

[andreas-gruenwald]

@ultramegatom Can you add a solution suggestion?

[ultramegatom]

@andreas-gruenwald
make an interface/abstract class and let the objectbrick itself decide what to return here