Summary: Fixes T11556. This was just missing an `implements ...`, which became necessary at some point even for classes that don't use much of the beahvior (ModularTransactions?).
Test Plan: Created a new test payment provider on a Phortune merchant.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11556
Differential Revision: https://secure.phabricator.com/D16471