Define `subnet.Info` structure of corresponding message from NeoFS API
protocol. Provide field getters and setters. Implement
`StableMarshal` / `StableSize` methods of binary encoding. Implement
`ToGRPCMessage` / `FromGRPCMessage` method for testing and
potential transport.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
