Add an opportunity for participants to pay fee for coordinator

[Velnbur]

Situation

There is no way for shuffle participants to pay a fee for sending transactions to the Ethereum network for coordinator. It makes no sense for the coordinator to maintain itself as it makes process killing.

Objective

Add an opportunity for participants to "thank" coordinator for its work.

Consideration

This could be some additional input and output in the transfer method which will go to coordinator.

Acceptance

A participants could pay fee for coordinator during shuffling process