Alerting: create wrapper for Alertmanager to enable org level isolation (#37320)

Introduces org-level isolation for the Alertmanager and its components.

Silences, Alerts and Contact points are not separated by org and are not shared between them.

Co-authored with @davidmparrott and @papagian

pkg/api/api.go

@@ -385,7 +385,7 @@ func (hs *HTTPServer) registerRoutes() {
 		})
 
 		apiRoute.Get("/alert-notifiers", reqEditorRole, routing.Wrap(
-			GetAlertNotifiers(hs.Alertmanager != nil && hs.Cfg.IsNgAlertEnabled())),
+			GetAlertNotifiers(hs.MultiOrgAlertmanager != nil && hs.Cfg.IsNgAlertEnabled())),
 		)
 
 		apiRoute.Group("/alert-notifications", func(alertNotifications routing.RouteRegister) {