Alerting: Receivers API (read only endpoints) (#81751)

* Add single receiver method

* Add receiver permissions

* Add single/multi GET endpoints for receivers

* Remove stable tag from time intervals

See end of PR description here: https://github.com/grafana/grafana/pull/81672

pkg/services/ngalert/api/api.go

@@ -64,6 +64,7 @@ type API struct {
 	StateManager         *state.Manager
 	AccessControl        ac.AccessControl
 	Policies             *provisioning.NotificationPolicyService
+	ReceiverService      *notifier.ReceiverService
 	ContactPointService  *provisioning.ContactPointService
 	Templates            *provisioning.TemplateService
 	MuteTimings          *provisioning.MuteTimingService
@@ -152,7 +153,11 @@ func (api *API) RegisterAPIEndpoints(m *metrics.API) {
 		hist:   api.Historian,
 	}), m)
 
-	api.RegisterNotificationsApiEndpoints(NewNotificationsApi(api.MuteTimings), m)
+	api.RegisterNotificationsApiEndpoints(NewNotificationsApi(&NotificationSrv{
+		logger:            logger,
+		receiverService:   api.ReceiverService,
+		muteTimingService: api.MuteTimings,
+	}), m)
 
 	// Inject upgrade endpoints if legacy alerting is enabled and the feature flag is enabled.
 	if !api.Cfg.UnifiedAlerting.IsEnabled() && api.FeatureManager.IsEnabledGlobally(featuremgmt.FlagAlertingPreviewUpgrade) {