[#1680] node/netmap: Support `MaintenanceModeAllowed` config in RPC

After recent changes network configuration stored in the Netmap contract
of the NeoFS Sidechain contains `MaintenanceModeAllowed` flag. There is
a need to support this value in Storage node application.

Make `NetmapService.NetworkInfo` RPC server of the storage node to set
`MaintenanceModeAllowed` flag according to corresponding value in the
Sidechain.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/netmap.go

@@ -404,6 +404,10 @@ func (n *netInfo) Dump(ver version.Version) (*netmapSDK.NetworkInfo, error) {
 			ni.DisableHomomorphicHashing()
 		}
 
+		if netInfoMorph.MaintenanceModeAllowed {
+			ni.AllowMaintenanceMode()
+		}
+
 		for i := range netInfoMorph.Raw {
 			ni.SetRawNetworkParameter(netInfoMorph.Raw[i].Name, netInfoMorph.Raw[i].Value)
 		}