get https://svc.blockdaemon.com/boss/v1/ethereum//withdrawable-amount/
Returns the total staked amount that can be withdrawn across a customer's 0x02 validators on the network with the given withdrawal address. Withdrawable amount per validator is calculated as 'withdrawable_amount = balance - minimum_deposit - pending_withdrawals'. Withdrawable amount does not include the balances of validators that can be exited and fully withdrawn.