Bug 32426: Patron::generate_userid - add plugin hook

Bug 32426: Patron::generate_userid - add plugin hook

The hook is called patron_generate_userid. How suitable.
The existing generate_userid tests and new tests with mocked
plugins are merged into a new test script.

Test plan:
Run t/db_dependent/Koha/Patron_generate_userid.t
Run t/db_dependent/Koha/Patrons.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>