Payment protocol: indicate whether or not a ZCE is optional

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In most cases, the user (or the user's wallet) should have the choice of whether or not to use a ZCE. We should consider some flag or other method for the merchant to indicate whether or not a ZCE is absolutely required.

Optional would be for circumstances where only that customer is inconvenienced – e.g. waiting for an online registration or gift card purchase.

Required would be for circumstances where the slow payment bothers other customers – a vending machine, an ATM, etc.