Bug 34211: Add +strings for GET /api/v1/biblios/:biblio_id/items

Bug 34211: Add +strings for GET /api/v1/biblios/:biblio_id/items

Allow to embed stringified version of attributes for the "Get items for
a biblio" endpoint

Test plan:
curl -u koha:koha --request GET 'http://localhost:8081/api/v1/biblios/47/items' --header "Content-Type: application/json" --header "x-koha-embed: +strings" | jq

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 2d27d02a4d63b3d3a322db18db8420693e986fdd)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>