Bug 27352: Add GET /biblios/:biblio_id/items

Bug 27352: Add GET /biblios/:biblio_id/items

This patch adds a route to fetch the items for a biblio. It relies
entirely on already existing methods.

To test:
1. Use your favourite REST tool
2. Play with the new route.
=> SUCCESS: It works as expected!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>