Bug 36503: Add a plugin hook after user authentication

Bug 36503: Add a plugin hook after user authentication

This plugin hook allows to change patron data or define the patron
based on the authenticated user.

To test: Run
prove t/db_dependent/Koha/Auth/Client.t

Sponsored-by: Karlsruhe Institute of Technology (KIT)
Signed-off-by: Thomas Klausner <domm@plix.at>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 911342ffec8f900e2b4e36fcb0208a7560bfd49c)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>