IHederaTokenService.sol - custom fee schedule detector

[littletarzan]

I propose adding a Hedera Smart Contract Service (HSCS) system contract function that allows a smart contract to determine whether a Hedera Token Service (HTS) token contains a customFeeSchedule and/or a non-empty customFeeScheduleKey. This information is important for some smart contracts to safely interact with tokens without risk of unintended token transfers caused by malicious fee schedules.

For a simple example, suppose any externally owned account (EOA) could associate tokens to a smart contract, and that smart contract is meant to have a balance of valuable tokens. A malicious fee schedule token could be associated and transferred to that contract. If, through normal operation, users could transfer tokens out of the smart contract, the malicious fee schedule token could be transferred as well as the balances of valuable tokens along with it, directly to an account they do not belong.

A possible IHederaTokenService.sol implementation to mitigate this could be

interface IHederaTokenService {
    function detectCustomFees(address token) external view returns (
        bool hasCustomFeeSchedule,
        bool hasCustomFeeScheduleKey
    );
}