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/ml/testing.go

@@ -0,0 +1,44 @@
+package ml
+
+import (
+	"time"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+
+	"github.com/grafana/grafana/pkg/api/response"
+)
+
+type FakeCommand struct {
+	Method     string
+	Path       string
+	Payload    []byte
+	Response   *backend.QueryDataResponse
+	Error      error
+	Recordings []struct {
+		From     time.Time
+		To       time.Time
+		Response response.Response
+		Error    error
+	}
+}
+
+var _ Command = &FakeCommand{}
+
+func (f *FakeCommand) DatasourceUID() string {
+	return "fake-ml-datasource"
+}
+
+func (f *FakeCommand) Execute(from, to time.Time, executor func(method string, path string, payload []byte) (response.Response, error)) (*backend.QueryDataResponse, error) {
+	r, err := executor(f.Method, f.Path, f.Payload)
+	f.Recordings = append(f.Recordings, struct {
+		From     time.Time
+		To       time.Time
+		Response response.Response
+		Error    error
+	}{From: from, To: to, Response: r, Error: err})
+
+	if err != nil {
+		return nil, err
+	}
+	return f.Response, f.Error
+}