Bug 26181: Disable override by default in /holds

Bug 26181: Disable override by default in /holds

This patch disables AllowHoldPolicyOverride by default in /holds. It
also adds a header that can be used to request the override explicitly.

Tests are added for this behaviour

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/holds.t
=> FAIL: Tests fail because the behaviour is not implemented
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>