dev/financial#77 ++ Make contribution_id mandatory for PaymentProcessor.pay, pass...

dev/financial#77 ++ Make contribution_id mandatory for PaymentProcessor.pay, pass incoieID

Replaces https://github.com/civicrm/civicrm-core/pull/15477 & also resolves https://lab.civicrm.org/dev/financial/issues/77

by requiring contribution_id for PaymentProcessor.pay as a way to ensure that it is only called after the order is created
per our preferred flow (people could still get past this but it feels like they would at least know they werre
hacking our process & take responsibility for any issues if it breaks or we suddenly start enforcing that
it is a valid contribution.

This also sets some of the recommended variables. Note that I had to use a few more lines to ensure we were always setting
contactID, contributionRecurID to an integer. I do think this stricter approach is better but it wound up
more verbose