SSE: Support hysteresis threshold expression (#70998)

* extend threshold command with second evaluator called `unloadEvaluator` 
* Introduce a new expression command Hysteresis and update Threshold unmarshaller to create the HysteresisCommand if the second eval
* add feature flag `recoveryThreshold`
* update unmarshal threshold command to not re-marshall because it breaks frame definition by shuffling the schema and data fields

pkg/expr/mathexp/types.go

@@ -13,6 +13,11 @@ type Results struct {
 	Error  error
 }
 
+// IsNoData checks whether the result contains NoData value
+func (r Results) IsNoData() bool {
+	return len(r.Values) == 0 || len(r.Values) == 1 && r.Values[0].Type() == parse.TypeNoData
+}
+
 // Values is a slice of Value interfaces
 type Values []Value