Get a biller's verifications

Returns the biller's verification history, latest first.

checks[] is ordered and open. Checks may be added, removed or reordered, so an integration must not switch exhaustively on key or fail on an unrecognised one.

A check that does not apply to a biller is left out of the list rather than sent with a status. A missing key means only that Adfin is not asking this biller for it right now.