Implement php api interface for payment methods data from server to client

[nerrad]

Currently payment methods would pass through and consume their own necessary data from server to client (api key, js initialization settings etc). This issue is for implementing a php api interface for payment methods to register data on the server to pass through to the payment registration client side.

The goal here is for payment methods to have a consistent interface for passing along (and implementing) that data eliminating duplication across all the payment method extensions.