Add Notification Manager to alertmanager webhook receiver

[benjaminhuo]

Signed-off-by: Benjamin [email protected]

Notification Manager is a K8s native notification management tool with multi-tenancy support. It can receive alerts from Alertmanager, and send notifications to various sinks defined by various CRDs. Furthermore, it has namespace-based tenant controller capability, notifications will be sent to the receiver CRDs who have the same namespace with notifications' namespace label.