Added support for getting the non-zero balances of an account

[mdawar]

This pull request adds the function OmitZeroBalances on GetAccountService to set the omitZeroBalances query parameter on the request.

Docs: https://developers.binance.com/docs/binance-spot-api-docs/rest-api#account-endpoints

By default all the balances will be returned, this param makes the API return the non zero balances of an account only.

Usage:

apiKey := os.Getenv("API_KEY")
secretKey := os.Getenv("SECRET_KEY")

client := binance.NewClient(apiKey, secretKey)

account, err := client.NewGetAccountService().OmitZeroBalances(true).Do(context.TODO())