Bug 33146: Add public items lookup route

Bug 33146: Add public items lookup route

This patch adds a /public equivilent to the item listing endpoint.

This allows us to search for an item by it's external_id (barcode).

Test plan
1. Apply patch
2. Perform a GET on /api/v1/public/items?external_id=some_barcode
3. Confirm that the above endpoint correctly returns items that should
   be visible in the OPAC

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Bug 33146: (QA follow-up) Do not delete all items in test

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Bug 33146: Allow embedding expanded coded values

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Bug 33146: (QA follow-up) Consistency with /biblios/:biblio_id/items

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Bug 33146: (QA follow-up) Make sure public API enabled for tests

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>