[#355] subnet: Implement types for the new messages

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>

refs/grpc/types.go

@@ -89,3 +89,10 @@ func (x *ChecksumType) FromString(s string) bool {
 
 	return ok
 }
+
+// SetValue sets subnet identifier in a base-10 integer format.
+func (x *SubnetID) SetValue(v uint32) {
+	if x != nil {
+		x.Value = v
+	}
+}