Add support for multicurrency

This is a suggestion to add multi-currency support to the Binance plugin if Woocommerce allows orders in different currencies. For example, in my shop I support both PLN and EUR – I’m using CURCY – WooCommerce Multi Currency plugin. The current version of the Binance plugin only allows you to set one fiat currency. A simple modification in the code allowed me to support multiple fiat currencies and during testing I see that the conversion works correctly with PLN as well. Of course, you can also add condition to check if the WC currency is supported on Binance side. For my shop where I only have two currencies, however, this is not necessary.

$req['fiatCurrency'] = $order->get_currency(); // replaced binance $currency for order currency