ot::BorderRouter::RoutingManager::RequestStop() method

Requests the Border Routing Manager to stop. If Border Routing Manager is running, calling this method immediately stops it and triggers the preparation and sending of a final Router Advertisement (RA) message on infrastructure interface which deprecates and/or removes any previously advertised PIO/RIO prefixes. If Routing Manager is not running (or not enabled), no action is taken. Note that this method does not change whether the Routing Manager is enabled or disabled (see `SetEnabled()`). It stops the Routing Manager temporarily. After calling this method if the device role gets changes (device gets attached) and/or the infra interface state gets changed, the Routing Manager may be started again.