[Feature] Add trading/currency support for GRT

Author: dougdellolio

CoinbasePro/Shared/Types/Currency.cs

@@ -49,6 +49,7 @@ public enum Currency
         FIL,
         AAVE,
         BNT,
-        SNX
+        SNX,
+        GRT
     }
 }

CoinbasePro/Shared/Types/ProductType.cs

@@ -255,6 +255,14 @@ public enum ProductType
         [EnumMember(Value = "SNX-EUR")]
         SnxEur,
         [EnumMember(Value = "SNX-GBP")]
-        SnxGbp
+        SnxGbp,
+        [EnumMember(Value = "GRT-USD")]
+        GrtUsd,
+        [EnumMember(Value = "GRT-BTC")]
+        GrtBtc,
+        [EnumMember(Value = "GRT-EUR")]
+        GrtEur,
+        [EnumMember(Value = "GRT-GBP")]
+        GrtGbp
     }
 }