module api: expose box_key_def_extract_key()

Unlike box_tuple_extract_key() it accepts a key_def structure, not
space_id, index_id pair.

Another difference from box_tuple_extract_key() is that this function
allows to pass so called multikey index. See commit 2.2.0-259-gf1d9f2575
('box: introduce multikey indexes in memtx') for details.

Note: The <multikey_idx> parameter is ignored on the backported version
of the patch on 1.10.

Part of #5273