Amount with int type may exceed max size on Ethereumn transaction

[shawn-cx-li]

Hi @eslavyansky

https://github.com/jazzserve/bitgo/blob/master/wallet-operations-basic.go#L177

int type has a max size of 2^63 -1 which is 9.22 * 10^18
According to the documentation on bitgo websize, we are sending the base unit for "sendcoins" API, which is "wei" on Ethereum. It will be problematic if we are sending over 9.23 eth from this SDK, becasue 9.23 eth = 9.23 * 10^18 wei and this number breaks the max size of int type.

Could you change it to string so it will be safe to send coins on Ethreumn?