Example chain-agnostic contract

[hashedone]

We are missing the oversimplified example of chain-agnostic contract (and interface). The intuition is to implement it as an Empty custom message contract, but it's not a proper way.

#399 issue is there to make sure the real-world examples are using the proper techniques to implement chain-agnostic contracts, but we also need a very simple example that reduces irrelevant boilerplate.