box: expose sort_order to C API

NO_CHANGELOG=see the previous commit

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

Looks like the `box_key_part_def_t` type isn't documented, but
`BOX_KEY_PART_DEF_SORT_ORDER` bit field was added to its `flags`
member. The possible values are:

`BOX_KEY_PART_DEF_SORT_ORDER_ASC`: default sort order.
`BOX_KEY_PART_DEF_SORT_ORDER_DESC`: reversed sort order.