ftx: add greek field support for GetFutureStats function (#864)

* ftx: update with greeks @romano thankyou

* ftx: fix nits

exchanges/ftx/ftx_test.go

@@ -230,6 +230,15 @@ func TestGetFutureStats(t *testing.T) {
 	if err != nil {
 		t.Error(err)
 	}
+
+	future, err := f.GetFutureStats(context.Background(), "BTC-MOVE-2021Q4")
+	if err != nil {
+		t.Error(err)
+	}
+
+	if future.Greeks == nil {
+		t.Fatal("no greeks returned for futures contract")
+	}
 }
 
 func TestGetFundingRates(t *testing.T) {

exchanges/ftx/ftx_types.go

@@ -163,6 +163,11 @@ type FutureStatsData struct {
 	PredictedExpirationPrice float64   `json:"predictedExpirationPrice"`
 	OpenInterest             float64   `json:"openInterest"`
 	StrikePrice              float64   `json:"strikePrice"`
+	Greeks                   *struct {
+		ImpliedVolatility float64 `json:"impliedVolatility"`
+		Delta             float64 `json:"delta"`
+		Gamma             float64 `json:"gamma"`
+	} `json:"greeks"`
 }
 
 // FundingRatesData stores data on funding rates