Bug 30180: Add 'placed' after_hold_hook

Bug 30180: Add 'placed' after_hold_hook

This patch adds a new 'after_hold_action' hook, that is called with the
'placed' action parameter.

To test:
1. Apply the unit tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Plugins/Holds_hooks.t
=> FAIL: Boo, the hook is not called
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>