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/bus/mock/mock.go

@@ -0,0 +1,11 @@
+package mock
+
+import (
+	"github.com/grafana/grafana/pkg/bus"
+	"github.com/grafana/grafana/pkg/infra/tracing"
+)
+
+func New() bus.Bus {
+	tracer := tracing.InitializeTracerForTest()
+	return bus.ProvideBus(tracer)
+}