Bug 16965: Add Koha::Objects->search_related

Bug 16965: Add Koha::Objects->search_related

In order to search on relations, we need this new method.

Test plan:
Confirm that the changes in Objects.t make sense and that the tests
pass.

Tested all 3 patches together, followed test plan, result OK
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>