Alerting: Add method to reset notification policy tree back to the default (#51934) (#52004)

* Define route and run codegen

* Wire up HTTP layer

* Update API layer and test fakes

* Implement reset of policy tree

* Implement service layer test and authorization bindings

* API layer testing

* Be more specific when injecting settings

(cherry picked from commit fce283d73e)

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

pkg/services/ngalert/api/generated_base_api_provisioning.go

@@ -40,6 +40,7 @@ type ProvisioningApiForkingService interface {
 	RoutePutMuteTiming(*models.ReqContext) response.Response
 	RoutePutPolicyTree(*models.ReqContext) response.Response
 	RoutePutTemplate(*models.ReqContext) response.Response
+	RouteResetPolicyTree(*models.ReqContext) response.Response
 }
 
 func (f *ForkedProvisioningApi) RouteDeleteAlertRule(ctx *models.ReqContext) response.Response {
@@ -156,6 +157,9 @@ func (f *ForkedProvisioningApi) RoutePutTemplate(ctx *models.ReqContext) respons
 	}
 	return f.forkRoutePutTemplate(ctx, conf, nameParam)
 }
+func (f *ForkedProvisioningApi) RouteResetPolicyTree(ctx *models.ReqContext) response.Response {
+	return f.forkRouteResetPolicyTree(ctx)
+}
 
 func (api *API) RegisterProvisioningApiEndpoints(srv ProvisioningApiForkingService, m *metrics.API) {
 	api.RouteRegister.Group("", func(group routing.RouteRegister) {
@@ -369,5 +373,15 @@ func (api *API) RegisterProvisioningApiEndpoints(srv ProvisioningApiForkingServi
 				m,
 			),
 		)
+		group.Delete(
+			toMacaronPath("/api/v1/provisioning/policies"),
+			api.authorize(http.MethodDelete, "/api/v1/provisioning/policies"),
+			metrics.Instrument(
+				http.MethodDelete,
+				"/api/v1/provisioning/policies",
+				srv.RouteResetPolicyTree,
+				m,
+			),
+		)
 	}, middleware.ReqSignedIn)
 }