feat: optional callback enabling fuel scripts to call simplevrf

[DarthBenro008]

Optional callback

• not every call is made by contract, the callback reverts if the caller isnt a contract
• making an optional callback which defaults to True if None is passed
• Fuel scripts can now call simplevrf without the contract reverting due to callback.

Feature requested by @IGI-111

[IGI-111]

I'm not sure I understand the logic behind making it an Option. This will still be incompatible with all previous usages, since the function selector hash includes the arguments.

If you want backwards compatibility, a better way would be to have a fn request_without_callback(seed: b256) -> u64.

If you're willing to break the interface however, I think making it an Option<b256> makes more sense, allowing the callback to be another contract than the caller:

// same behavior as current usage
let rid = request(seed, Some(CONTRACT_ID));
// no callback
let rid = request(seed, None);
// call another contract
let rid = request(seed, Some(other_contract_id));