Bug 30674: x-koha-override should use collectionFormat: csv

Bug 30674: x-koha-override should use collectionFormat: csv

This patch makes this header parameter rely on the OpenAPI spec to
validate and document the available options.

Right now the only place is in POST and PUT /holds.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/holds.t \
           t/db_dependent/api/v1/auth_authenticate_api_request.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests still pass!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>