Bug 24615: Make object.search helper also order by embedded columns

Bug 24615: Make object.search helper also order by embedded columns

With this patch REST API request can order results by embedded columns.

Full path to the column must be given for it to work.
For example: If you are on biblio endpoint and you want to order by holding patron's card number you could

> GET /biblio/1?_order_by=item.holds.cardnumber HTTP/1.1
> x-koha-embed: item.holds

To test:
1. apply this patch
2. prove t/db_dependent/Koha/REST/Plugin/Objects.t

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