Bug 16825: Add API route for getting an item

Bug 16825: Add API route for getting an item

GET /api/v1/items/{item_id} Gets one Item

This patch adds route to get one item from koha.items table.

To test:
1. Apply patch
2. Open a browser tab on Koha staff and log in (to create CGISESSID
   cookie).
3. Send GET request to http://yourlibrary/api/v1/items/YYY
   where YYY is an existing itemnumber.
4. Make sure the returned data is correct.
5. Run unit tests in t/db_dependent/api/v1/items.t

Sponsored-by: Koha-Suomi Oy
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Johanna Raisa <johanna.raisa@gmail.com>
Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>