Bug 27760: Add stash_overrides helper

Bug 27760: Add stash_overrides helper

This patch adds a simple helper that reads the x-koha-override request
header, and processes it to stash a hashref with the passed overrides.

No check on the overrides themselves is done, as they should be
validated using the OpenAPI spec.

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

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>