Expressions: Move GEL into core as expressions (#29072)

* comes from grafana/gel-app
* remove transform plugin code
* move __expr__ and -100 constants to expr pkg
* set OrgID on request plugin context
* use gtime for resample duration
* in resample, rename "rule" to "window", use gtime for duration, parse duration before exec
* remove gel entry from plugins-bundled/external.json
which creates an empty array for plugins

pkg/expr/service.go

@@ -0,0 +1,39 @@
+package expr
+
+import (
+	"context"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+)
+
+// DatasourceName is the string constant used as the datasource name in requests
+// to identify it as an expression command.
+const DatasourceName = "__expr__"
+
+// DatasourceID is the fake datasource id used in requests to identify it as an
+// expression command.
+const DatasourceID = -100
+
+// Service is service representation for expression handling.
+type Service struct {
+}
+
+// BuildPipeline builds a pipeline from a request.
+func (s *Service) BuildPipeline(queries []backend.DataQuery) (DataPipeline, error) {
+	return buildPipeline(queries)
+}
+
+// ExecutePipeline executes an expression pipeline and returns all the results.
+func (s *Service) ExecutePipeline(ctx context.Context, pipeline DataPipeline) (*backend.QueryDataResponse, error) {
+	res := backend.NewQueryDataResponse()
+	vars, err := pipeline.execute(ctx)
+	if err != nil {
+		return nil, err
+	}
+	for refID, val := range vars {
+		res.Responses[refID] = backend.DataResponse{
+			Frames: val.Values.AsDataFrames(refID),
+		}
+	}
+	return res, nil
+}