sendNewValidatorTransaction method

Sends a `new_validator` transaction to the network. You need to provide the address of a basic account (the sender wallet) to pay the transaction fee and the validator deposit. 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: \"\" = Set the signal data field to None. \"0x29a4b...\" = Set the signal data field to Some(0x29a4b...).