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/api/authorization_test.go

@@ -49,7 +49,7 @@ func TestAuthorize(t *testing.T) {
 		}
 		paths[p] = methods
 	}
-	require.Len(t, paths, 40)
+	require.Len(t, paths, 41)
 
 	ac := acmock.New()
 	api := &API{AccessControl: ac}