Bug 33690: Convert to x-koha-override

Bug 33690: Convert to x-koha-override

This patch converts the existing work to use x-koha-override instead of
introducing an entirely new header and we default to the system
preference value if the override isn't passed.

I believe this is more consistent with our approach in other areas of
koha to date.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>