[Add] 增加OkexFutureSymbol类以表示可支持的合约（无法直接从API查询）

vnpy/api/okexfuture/OkexFutureApi.py

@@ -13,6 +13,15 @@ class _OkexFutureCustomExtra(object):
         self.onFailed = onFailed
         self.onSuccess = onSuccess
         self.extra = extra
+        
+        
+########################################################################
+class OkexFutureSymbol(Enum):
+    BTC = 'btc_usd'
+    LTC = 'ltc_usd'
+    ETH = 'eth_usd'
+    ETC = 'etc_usd'
+    BCH = 'bch_usd'
 
 
 ########################################################################

vnpy/api/okexfuture/__init__.py

@@ -0,0 +1,2 @@
+from .OkexFutureApi import OkexFutureSymbol, OkexFutureContractType, OkexFutureOrder, OkexFutureOrderStatus, OkexFuturePosition, \
+    OkexFuturePositionDetail, OkexFuturePriceType, OkexFutureRestClient, OkexFutureUserInfo