Add subnet-related node attributes

[cthulhu-rider]

• Blocked by Define structure of the information about subnetwork #180.

Summary

Storage nodes can register on system subnets (at least one). The subnets that a node intends to enter are specified in the node attributes.

Task

• Add new class of node attributes (netmap.NodeInfo.Attibute message) with __NEOFS__SUBNET_%s key format. Each attribute is related to a specific subnet by string ID. The value reflects the intention to enter (True) / exit (False) the subnet.
• Mention in docs special behavior of zero subnet. Missing attribute of zero subnet equivalent to True value.