Extract assignPaymentFields

[mattwire]

Overview

Proof of concept towards proposed refactor per #13762 (comment)

@eileenmcnaughton I'm thinking this function should be in a new trait that can be shared between CRM_Contribute_Form_ContributionBase and CRM_Event_Form_Registration. Alternatives are we inherit from contributionbase for registration form or make this function static. Thoughts?

[civibot]

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[eileenmcnaughton]

@mattwire I was thinking the new FrontEndPaymentFormTrait would be the place. If this is a straight extraction I'm happy to review & merge it even if it later moves again

[mattwire]

@eileenmcnaughton Yes, it's a straight extraction. No changes. So we can review, merge and then move to trait + cleanup / make generic enough so it works for event registration

[eileenmcnaughton]

test this please

[eileenmcnaughton]

I redid this locally & got to the same result - merge on pass