Alerting: Tiny refactor on the eval and schedule packages (#66130)

* Alerting: Tiny refactor on the eval and schedule packages

two very small things:

- We had a constructor on something called a `Context` which is not a `context.Context` so let's just name that constructor `NewContext`
- The user that we use to run query evaluations is the same (with some variation) abstract it to a function so that it can be re-used when necessary.

* Update pkg/services/ngalert/schedule/schedule.go

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

* Update pkg/services/ngalert/schedule/schedule.go

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

---------

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

pkg/services/ngalert/api/api_testing.go

@@ -52,7 +52,7 @@ func (srv TestingApiSrv) RouteTestGrafanaRuleConfig(c *contextmodel.ReqContext,
 		Condition: body.GrafanaManagedCondition.Condition,
 		Data:      queries,
 	}
-	ctx := eval.Context(c.Req.Context(), c.SignedInUser)
+	ctx := eval.NewContext(c.Req.Context(), c.SignedInUser)
 
 	conditionEval, err := srv.evaluator.Create(ctx, evalCond)
 	if err != nil {
@@ -128,7 +128,7 @@ func (srv TestingApiSrv) RouteEvalQueries(c *contextmodel.ReqContext, cmd apimod
 	if len(cmd.Data) > 0 {
 		cond.Condition = cmd.Data[0].RefID
 	}
-	evaluator, err := srv.evaluator.Create(eval.Context(c.Req.Context(), c.SignedInUser), cond)
+	evaluator, err := srv.evaluator.Create(eval.NewContext(c.Req.Context(), c.SignedInUser), cond)
 
 	if err != nil {
 		return ErrResp(http.StatusBadRequest, err, "Failed to build evaluator for queries and expressions")