Alerting: Optimize prometheus api permission checks (#106299)

* Alerting: Optimize prometheus api permission checks

This improves the performance of the Prometheus API by performing the permission checks for rule read permission in a folder upfront, rather than checking permissions for each rule group individually. This reduces the number of permission checks and should speed up the API response time.

* refactor vars

---------

Co-authored-by: Konrad Lalik <konradlalik@gmail.com>

pkg/services/ngalert/api/testing.go

@@ -129,6 +129,10 @@ var _ ac.AccessControl = &recordingAccessControlFake{}
 type fakeRuleAccessControlService struct {
 }
 
+func (f *fakeRuleAccessControlService) HasAccessInFolder(ctx context.Context, user identity.Requester, folder models.Namespaced) (bool, error) {
+	return true, nil
+}
+
 func (f fakeRuleAccessControlService) HasAccessToRuleGroup(ctx context.Context, user identity.Requester, rules models.RulesGroup) (bool, error) {
 	return true, nil
 }