Analyze feasible proof-of-payment methods

[s-tikhomirov]

In a client-server interaction in Store, after the price is agreed upon (via #51 and #52), the client needs to prove the payment to the server. Possible ways to do that broadly fall into two categories:

1. the client pays just-in-time and provides a proof of a transaction directly corresponding to this request (the transaction may be on-chain, in a payment channel, in a rollup, etc);
2. the client pays in advance and provides a service credential, or some (zero-knowledge?) proof that it belongs to the set of eligible users.

Choose the appropriate method a) for the PoC; b) for the long-term vision of the protocol.