Add example of how to call a Solang Solidity contract from ink!

[cmichi]

ToDo

• [ ] Solang should generate the same selectors as ink! (@xgreenx mentioned this a while ago, is there by chance already an issue in the Solang repo?)
• [ ] Solang should generate contract-interface.rs with a trait that exposes the contract methods.
• [ ] The example should demonstrate cross-contract calls.

Blocked by https://github.com/paritytech/cargo-contract/issues/526.

[HCastano]

@xermicus assigning you since you're in the best position to (eventually) close this