SSE: Support for ML query node (#69963)

* introduce a new node-type ML and implement a command outlier that uses ML plugin as a source of data.
* add feature flag mlExpressions that guards the feature

pkg/expr/nodes.go

@@ -47,6 +47,7 @@ type baseNode struct {
 type rawNode struct {
 	RefID      string `json:"refId"`
 	Query      map[string]interface{}
+	QueryRaw   []byte
 	QueryType  string
 	TimeRange  TimeRange
 	DataSource *datasources.DataSource