Alerting: Notifications Routes API (#91550)

* Introduce new models RoutingTree, RouteDefaults and Route and api-server to serve them that is backed by provisioning notification policy service.

* update method UpdatePolicyTree of notification policy service to return route and new version

* declare new actions alert.notifications.routes:read and alert.notifications.routes:write and two corresponding fixed roles.

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Co-authored-by: Tom Ratcliffe <tom.ratcliffe@grafana.com>
Co-authored-by: Matthew Jacobson <matthew.jacobson@grafana.com>

pkg/services/cloudmigration/cloudmigrationimpl/snapshot_mgmt_alerts_test.go

@@ -258,6 +258,6 @@ func updateNotificationPolicyTree(t *testing.T, ctx context.Context, service *Se
 		Routes:   []*definition.Route{&child},
 	}
 
-	err := service.ngAlert.Api.Policies.UpdatePolicyTree(ctx, user.GetOrgID(), tree, "", "")
+	_, _, err := service.ngAlert.Api.Policies.UpdatePolicyTree(ctx, user.GetOrgID(), tree, "", "")
 	require.NoError(t, err)
 }