Bug 24356: Make objects.search prefetch embedded relations

Bug 24356: Make objects.search prefetch embedded relations

This patch makes the Koha::Object(s) derived classes expose information
about prefetch-able relations. This is then used by a new helper to
generate the prefetch information for the DBIC query.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Object* \
           t/db_dependent/Koha/REST/Plugin/Objects.t \
           t/Koha/REST/Plugin/Query.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>