Define structure of the information about subnetwork

[cthulhu-rider]

Summary

Information about subnetwork is stored and trasmitted between NeoFS components. It must have strict binary format and support
feature extensibility without losing backward compatibility. NeoFS API protocol messages are perfectly suited for this.

Task

Define SubnetInfo message structure (it can be placed in dedicated subnet package). It must consist of ID and owner ID (refs.OwnerID) fields. ID should be a separate type in order to describe its binary and string format. For now it should carry single numerical field. Both fields are required.

Note

ID definition is also required in #179.