NeoFS storage node can participate in a subnet group (at least one).
According to NeoFS API V2 protocol, subnets are entered and exited through
the attributes of the node. We should provide functionality for conveniently
setting and reading attributes based on the needs of the network.
Define `NodeSubnetInfo` type which groups information about the subnet
reflected in `NodeInfo`. Implement `WriteSubnetInfo` function which writes
`SubnetInfo` data to `NodeInfo`. It will be used to prepare a request for
registration on the NeoFS network. Implement `IterateSubnets` function which
allows to iterate over all subnets of the node. Moreover, it allows you to
remove a subnet from the `NodeInfo` right during iterative traversal.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
