Bug 28157: Add handling for the x-koha-library header

Bug 28157: Add handling for the x-koha-library header

This patch introduces a new header handling. The key idea is that on
Koha's base classes there's broad usage of C4::Context->userenv to
determine the current library and make decisions based on that.

API requests, on the other hand, might not be tied to sessions
(stateless) which is the way current library is retrieved. So we need a
way to properly specify what library the request is trying to act as
coming from.

To test:
1. Apply this patchset
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: Jonathan Druart <jonathan.druart@bugs.koha-community.org>