Create update validator transaction

Returns a serialized `update_validator` transaction. You need to provide the address of a basic account (the sender wallet) to pay the transaction fee. Since JSON doesn't have a primitive for Option (it just has the null primitive), we can't have a double Option. So we use the following work-around for the signal data: null = No change in the signal data field. \"\" = Change the signal data field to None. \"0x29a4b...\" = Change the signal data field to Some(0x29a4b...).