Alerting: rule backtesting API (#57318)

* Implement backtesting engine that can process regular rule specification (with queries to datasource) as well as special kind of rules that have data frame instead of query.
* declare a new API endpoint and model
* add feature toggle `alertingBacktesting`

pkg/services/ngalert/backtesting/eval_query.go

@@ -0,0 +1,27 @@
+package backtesting
+
+import (
+	"context"
+	"time"
+
+	"github.com/grafana/grafana/pkg/services/ngalert/eval"
+)
+
+// QueryEvaluator is evaluator of regular alert rule queries
+type queryEvaluator struct {
+	eval eval.ConditionEvaluator
+}
+
+func (d *queryEvaluator) Eval(ctx context.Context, from, to time.Time, interval time.Duration, callback callbackFunc) error {
+	for now := from; now.Before(to); now = now.Add(interval) {
+		results, err := d.eval.Evaluate(ctx, now)
+		if err != nil {
+			return err
+		}
+		err = callback(now, results)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}