Raise on raw modifications of box.cfg values

```sh
tarantool
> box.cfg{}
> box.cfg.read_only
false
> box.cfg.read_only = true
> box.cfg.read_only
true
```

I'd expect 3rd command should raise and cfg table at all would be readonly

Updated (25.01.2023): RFC - https://www.notion.so/Raw-modifications-of-box-cfg-values-9f440d321f984389a22bcb3f81366ea7