Not requiring prepayment

[nickevansuk]

In line with prepayment proposal openactive/modelling-opportunity-data#98, allow for a booking to be completed without payment being taken, if prepayment != https://openactive.io/Required.

Suggest allowing this via a payment of 0, to complete the booking without taking payment.

This lowers the barrier to spec implementation, as open booking can be implemented without a payment method, with payment taken when the participant arrives.

[ldodds]

I think the revised flow will handle this now?

[nickevansuk]

The revised flow in the current specification handles this, and it is further refined via #131