Create example contract utilising ZK-functions provided by host

[miloszm]

Summary

Dusk virtual machine supports privacy yet we do not have an example contract that would utilise ZK proof facility provided by the host. Such example contract would showcase this feature of our protocol.

Possible solution design or implementation

Implement a simple yet convincing real life example business case of cooperation of two contracts which do not trust each other and which perform a business interaction in a zero-knowledge manner, utilising the power of provided by the host ZK proving function(s).

Additional context

Business case should be understandable for everyone without the need to have a business background.

[HDauven]

@miloszm @ureeves can we state that this issue is resolved by Add zk proof verification test in Piecrust or do we want something more extensive?

[HDauven]

Already done here: dusk-network/piecrust#256