Alerting: Add first Grafana reserved label grafana_folder (#50262)

* Alerting: Add first Grafana reserved label g_label

g_label holds the title of the folder container the alert. The intention of this label
is to use it as part of the new default notification policy groupBy.

* Add nil check on updateRule labels map

* Disable gocyclo lint on schedule.ruleRoutine

will remove later in a separate refactoring PR to reduce complexity.

* Address doc suggestions

* Update g_folder for rules in folder when folder title changes

* Remove global bus in FolderService

* Modify tests to fit new common g_folder label

* Add changelog entry

* Fix merge conflicts

* Switch GrafanaReservedLabelPrefix from `g_` to `grafana_`

pkg/events/events.go

@@ -62,3 +62,11 @@ type DataSourceCreated struct {
 	UID       string    `json:"uid"`
 	OrgID     int64     `json:"org_id"`
 }
+
+type FolderUpdated struct {
+	Timestamp time.Time `json:"timestamp"`
+	Title     string    `json:"name"`
+	ID        int64     `json:"id"`
+	UID       string    `json:"uid"`
+	OrgID     int64     `json:"org_id"`
+}