- Add `MAINTENANCE_MODE` status code. It should be returned on every public API call if a Node is in maintenance mode. - Add `MAINTENANCE` [Node state](https://github.com/nspcc-dev/neofs-api/blob/4de2570e1a3dfc9633bbf02daf74753e7ba46b69/netmap/types.proto#L226).