[ByeBye Spree] Move Payment model to OFN

[luisramos0]

What we should change and why (this is tech debt)

Move Payment model and processing class to OFN and merge with decorators.

These files:

• core/app/models/spree/payment.rb MERGE WITH DECORATOR
• spec/models/spree/payment_spec.rb MERGE WITH OFN VERSION (or ignore)
• core/app/models/spree/payment/processing.rb COPY

Context

Part of #4826

Impact and timeline

This will enable us to stop working on top of decorators and refactor the Spree code to a better version (our coding standards are better)

[luisramos0]

@sauloperez assigned to you as this is already done in your PR #5678