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>

api/v1/swagger/paths/biblios.yaml

@@ -346,6 +346,16 @@
     summary: Get items for a biblio
     parameters:
       - $ref: "../swagger.yaml#/parameters/biblio_id_pp"
+      - name: x-koha-embed
+        in: header
+        required: false
+        description: Embed list sent as a request header
+        type: array
+        items:
+          type: string
+          enum:
+            - +strings
+        collectionFormat: csv
       - $ref: "../swagger.yaml#/parameters/match"
       - $ref: "../swagger.yaml#/parameters/order_by"
       - $ref: "../swagger.yaml#/parameters/page"