Add support for multi-contract contract calls

[digorithm]

This PR enables users to call contracts that are calling other external contracts. It introduces a chain method .set_contracts(&[...]) to do so. Like this:

let response = my_contract
    .my_method(...)
    .set_contracts(&[another_contract_id]) // Add this to set the external contract
    .call()
    .await
    .unwrap();

Closes #59.

[adlerjohn]

This is why moving all these docs into The Fuels-rs Book is a good idea: you could have them as real files then just include them.

[iqdecay]

Nitpicks