Bug 40542: Add strings embed to holds-related endpoints

This patch makes holds endpoints be allowed to embed strings.

To test:
1. Use your favourite REST tool to hit /api/v1/holds
2. Add the `x-koha-embed: +strings` header.
=> FAIL: The API rejects because it is not equiped with the feature.
3. Apply this patches
4. Run:
   $ ktd --shell
  k$ yarn api:bundle
  k$ koha-plack --restart kohadev
5. Repeat 2
=> SUCCESS: It works!
6. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit e90bc50209)
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>

api/v1/swagger/definitions/hold.yaml

@@ -135,5 +135,10 @@ properties:
       - object
       - "null"
     description: Pickup library
+  _strings:
+    type:
+      - object
+      - "null"
+    description: A list of stringified coded values
 
 additionalProperties: false

api/v1/swagger/paths/holds.yaml

@@ -101,6 +101,7 @@
         items:
           type: string
           enum:
+            - +strings
             - cancellation_requested
             - biblio
             - deleted_biblio

api/v1/swagger/paths/patrons_holds.yaml

@@ -28,6 +28,7 @@
         items:
           type: string
           enum:
+            - +strings
             - cancellation_requested
             - biblio
             - deleted_biblio