[#1681] morph/netmap: Support `MaintenanceModeAllowed` config

After recent changes in the NeoFS API protocol network configuration
contains `MaintenanceModeAllowed` boolean flag. There is a need to
support the config value in all NeoFS applications.

Provide `Client.MaintenanceModeAllowed` method which read the config
from the Sidechain. Extend `NetworkConfiguration` structure with
`MaintenanceModeAllowed` field.

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

pkg/morph/client/netmap/config.go

@@ -22,6 +22,7 @@ const (
 	irCandidateFeeConfig          = "InnerRingCandidateFee"
 	withdrawFeeConfig             = "WithdrawFee"
 	homomorphicHashingDisabledKey = "HomomorphicHashingDisabled"
+	maintenanceModeAllowedConfig  = "MaintenanceModeAllowed"
 )
 
 // MaxObjectSize receives max object size configuration
@@ -141,6 +142,15 @@ func (c *Client) WithdrawFee() (uint64, error) {
 	return fee, nil
 }
 
+// MaintenanceModeAllowed reads admission of "maintenance" state from the
+// NeoFS network configuration stored in the Sidechain. The admission means
+// that storage nodes are allowed to switch their state to "maintenance".
+//
+// By default, maintenance state is disallowed.
+func (c *Client) MaintenanceModeAllowed() (bool, error) {
+	return c.readBoolConfig(maintenanceModeAllowedConfig)
+}
+
 func (c *Client) readUInt64Config(key string) (uint64, error) {
 	v, err := c.config([]byte(key), IntegerAssert)
 	if err != nil {
@@ -246,6 +256,8 @@ type NetworkConfiguration struct {
 
 	HomomorphicHashingDisabled bool
 
+	MaintenanceModeAllowed bool
+
 	Raw []RawNetworkParameter
 }
 
@@ -312,6 +324,8 @@ func (c *Client) ReadNetworkConfiguration() (NetworkConfiguration, error) {
 			res.WithdrawalFee = bytesToUint64(value)
 		case homomorphicHashingDisabledKey:
 			res.HomomorphicHashingDisabled = bytesToBool(value)
+		case maintenanceModeAllowedConfig:
+			res.MaintenanceModeAllowed = bytesToBool(value)
 		}
 
 		return nil