Implement PhabricatorApplicationTransactionInterface on PhortunePaymentProviderConfig

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