Allow filtering Payment Gateway, Shipping Methods and Integration form fields

Developers should be able to filter the form fields of gateways, shipping methods and other settings. Rationale: sometimes you need to add custom configuration for a payment gateway (or other setting). 

For example - I am working with an integration where we need to add an external ID field to every payment gateway.
Also, there is shipping method plugin that I need to extend with some extra options for my special case.

This PR adds the required filters to each built-in Integration, Payment Gateway and Shipping Method. This however, of course, is not not enough as there are a gazillion of external plugins/extensions that need to address this. For this reason, I suggest announcing this change to all extension developers and politely ask them to update their code. I think this is the least we can do to make this change more global.