* Implement `NodeInfo.EnterSubnet` method which adds node's subnet.
* Implement `NodeInfo.IterateSubnets` which behaves similar to
eponymous function from neofs-api-go/v2.
* Implement `BelongsToSubnet` which checks if `NodeInfo` describes the
node belonging to a specific subnet.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
