Bug 33080: Introduce objects.search_rs, objects.find_rs and objects.to_api

Bug 33080: Introduce objects.search_rs, objects.find_rs and objects.to_api

This patch introduces the mentioned helpers, and reimplements
objects.search and objects.find in terms of them.

To test:
1. Apply patch
2. restart_all
3. Check that any API that uses objects.search helper is still running
   (GET /api/v1/items, GET /api/v1/holds, etc)
4. prove t/db_dependent/Koha/REST/Plugin/Objects.t

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>