Optionalgr4vyOptionalproviderThe payment provider instance this session belongs to, straight from
POST /checkout-sessions/:id/card-session.
Either name works. providerId is the documented one; gr4vyId is the
original and is still accepted, because the package shipped with it and
dropping it would break consumers already passing the response through.
The Task 2
POST /checkout-sessions/:id/card-sessionresponse shape.