post
https://apis.kentro.live/store/v1/payments.save
Starts saving a card: with opaqueData and cardMetadata (Authorize.net) the card is stored at once; on Stripe the answer is state: "action" with the hosted setup page in redirectUrl. billing is the billing address; type is general, default or backup. To change a saved method send its id and expectedVersion. Idempotent per idempotencyKey; follow a change that is not complete with payments.status.
Who may call it: A signed-in buyer (X-Storefront-Buyer); without one the answer is 401 UNAUTHORIZED.
