Alerting: Migrate notifications API to app-platform SDK application (#104424)

* introduce alerting notification app
* move code as is and remove from old registry
* update api server registration
* update make file and remove unnecessary args, copy some useful make commands from dashboards
* update codeowners

* move constants inside module and remove dependency from grafana
* add support for selectors to the app builder

pkg/registry/apps/alerting/notifications/routingtree/storage.go

@@ -0,0 +1,16 @@
+package routingtree
+
+import (
+	"k8s.io/apiserver/pkg/registry/rest"
+
+	grafanarest "github.com/grafana/grafana/pkg/apiserver/rest"
+	"github.com/grafana/grafana/pkg/services/apiserver/endpoints/request"
+)
+
+func NewStorage(legacySvc RouteService, namespacer request.NamespaceMapper) grafanarest.Storage {
+	return &legacyStorage{
+		service:        legacySvc,
+		namespacer:     namespacer,
+		tableConverter: rest.NewDefaultTableConvertor(ResourceInfo.GroupResource()),
+	}
+}