Function query_crypto_max_buy_sell_qty 

pub async fn query_crypto_max_buy_sell_qty(
    backend: &BackendConn,
    ctx: &CryptoAccountContext,
    account_market: u32,
    order_type: u32,
    coin: &str,
    currency: &str,
    price: Option<String>,
    order_id: Option<String>,
) -> Result<CryptoMaxBuySellQtyInfo>

Query crypto max buy/sell quantity through CMD20622.

C++ NNProto_Trd_MaxQtyCrypto.cpp:14-38,49-78 builds crypto_risk::GetMaxBuySellReq from account context, cc_origin, and cc_destination.