Bug 18120: Add '/acquisitions/vendors' endpoint

Bug 18120: Add '/acquisitions/vendors' endpoint

This patch introduces an /acquisitions/vendors endpoint.
To test:
- Apply the patch
- Run:
  $ sudo koha-shell kohadev
 k$ prove t/db_dependent/api/v1/acquisitions_vendors.t
=> SUCCESS: Tests pass
- Sign off :-D

Sponsored-by: ByWater Solutions
Signed-off-by: Matthias Meusburger <matthias.meusburger@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>