box: expose sort_order to lua key_def API

* Add ability to specify sort_order when creating a key def via
  lua key_def API.
* Update key_def.totable to also emit the sort order.

NO_CHANGELOG=see the previous commit

@TarantoolBot document
Title: key_def API update - added support for key part sort order.

The part sort order is specified the same way as in index creation:

```
key_def.new({{1, 'unsigned', sort_order = 'desc'}})
```

The sort order is now taken into account in comparison functions.

Affected documentation: [link](https://www.tarantool.io/en/doc/latest/reference/reference_lua/key_def/)