added Gate.io ExchangeAPI implementation (#636)

* Gate.io: GetMarketSymbols

* Gate.io: GetMarketSymbolsMetadata

* Gate.io: GetTickersAsync

* Gate.io: GetRecentTradesAsync

* Gate.io: GetTickerAsync

* Gate.io: GetOrderBookAsync

* Gate.io: GetCandles

* Gate.io: PlaceOrder

* Gate.io: PlaceOrders

* Gate.io: CancelOrder

* Gate.io: GetOpenOrderDetails

* Gate.io: GetOrderDetails

* Gate.io: GetCompletedOrderDetails

* update API definition

Author: gajewskiArek