Add payment processor method to add a js script to a page

This is an extra proposed method on the base payment processor class. The intention is that the payment processor would no longer need to implement a form hook to add script to a page when the processor appears.

$processor->addScript()

Not sure if it requires any parameters? Perhaps we should pass an array on the basis we are likely to come up with something later & changing signature on inherited functions could cause problems

The default class would contain an empty version of this function.

We would need to find where forms are built & ensure this function is called during building.