Fixed MaxWithdrawQuantity field for CoinM GetBalances() return type

[GeorgeF0]

Hi!

I've split GetBalances return type for CoinM into its own DTO and fixed MaxWithdrawQuantity to look for a JSON field called withdrawAvailable as per the docs here: https://binance-docs.github.io/apidocs/delivery/en/#futures-account-balance-user_data For some reason it's different than the usual maxWithdrawAmount field that they use in pretty much all other endpoints!

[JKorf]

Appreciate the PR. Just wondering if the Coin balances class couldn't be a derived type from a comon Futures balances class? That way people could implement shared code for both USDT and Coin balance processing