pay
Needs to be called when the checkout is state CheckoutState.REQUEST_PAYMENT_METHOD.
When successful the state will switch to CheckoutState.WAIT_FOR_APPROVAL which waits for a event of the backend that the payment has been confirmed or denied.
Possible states after receiving the event:
CheckoutState.PAYMENT_APPROVED CheckoutState.DENIED_BY_PAYMENT_PROVIDER
Parameters
paymentMethod
the payment method to pay with
paymentCredentials
may be null if the payment method requires no payment credentials