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/tooling/definitions/provisioning_policies.go

@@ -19,6 +19,16 @@ package definitions
 //       202: Ack
 //       400: ValidationError
 
+// swagger:route DELETE /api/v1/provisioning/policies provisioning stable RouteResetPolicyTree
+//
+// Clears the notification policy tree.
+//
+//     Consumes:
+//     - application/json
+//
+//     Responses:
+//       202: Ack
+
 // swagger:parameters RoutePutPolicyTree
 type Policytree struct {
 	// The new notification routing tree to use