Bug 27931: Add GET /items/:item_id/pickup_locations

Bug 27931: Add GET /items/:item_id/pickup_locations

This patch adds the mentioned route. It does so by:
- Adding the new path in paths.json
- Adding the full route spec in items.json
- Adds a controller method that takes care of the task

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/items.t
=> SUCCESS: Tests pass!
3. Play with your favourite REST tool. Pay special care to
   the AllowHoldPolicyOverride syspref and the expected behaviors.
=> SUCCESS: All works as expected
4. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>