Add a runtime API to simulate max net tx cost for the end user (fees - refund)

Currently, there is no way to predict the exact refund a user will receive.

Wallet developers need to accurately predict the refund so they can show users their actual final cost.

To achieve this, we need to add a runtime API to simulate the fees and the refund and return the net cost.

Related discussion on the Duniter forum: https://forum.duniter.org/t/taxe-remboursement-et-longueur-des-messages-de-tx/13161/3