Kraken - confirm trading agreement for German residents, see https://support.kraken.com/hc/en-us/articles/360000920026-Trading-agreement-required-error (#573)

Co-authored-by: d2ro <d2ro@users.noreply.github.com>

Author: d2ro

src/ExchangeSharp/API/Exchanges/Kraken/ExchangeKrakenAPI.cs

@@ -719,6 +719,7 @@ protected override async Task<ExchangeOrderResult> OnPlaceOrderAsync(ExchangeOrd
                 { "type", (order.IsBuy ? "buy" : "sell") },
                 { "ordertype", order.OrderType.ToString().ToLowerInvariant() },
                 { "volume", order.RoundAmount().ToStringInvariant() },
+                { "trading_agreement", "agree" },
                 { "nonce", nonce }
             };
             if (order.OrderType != OrderType.Market)