[SDK] Add specific class to represent Gas

[claytonneal]

At the moment signature of GasModule.estimateGas(...) is

interface EstimateGasResult {
    /**
     * The total gas cost estimation of the transaction.
     */
    totalGas: number;

    /**
     * Boolean indicating whether the transaction reverted or not.
     */
    reverted: boolean;

    /**
     * Decoded Solidity revert reasons for each clause.
     * If the n-th clause reverted, then the n-th element of this array will be the decoded revert reason for the n-th clause.
     *
     * @note revertReasons will be undefined if the transaction did not revert.
     */
    revertReasons: Array<string | bigint>;

    /**
     * The error message produced by the Virtual Machine.
     *
     * @note vmErrors will be undefined if the transaction did not revert.
     */
    vmErrors: string[];
}

totalGas is a number, and documentation says that it's a cost, which makes people believe that it can be used directly.

Unfortunately, it's a value that can be converted at the moment into VTHO by fixed means and in future releases of protocol it can be dynamic fee market depending on how full block is.

In order to make it clear that it's not a number that can be fit directly into VTHO most likely different unit of measure have to be introduced that might allow for conversion into the price token (given choice of the model).