Bug 34611: Add pseudonymize routine to Koha::Statistics

Bug 34611: Add pseudonymize routine to Koha::Statistics

This patch adds a new routine to pseudonymize a statistic and adjusts
C4::Stats to use this new routine.

Additionally Koha::PseudonymizedTransaction->new_from_statistic is updated to check for
the existence of objects before using them (in the case of older stats where things may have been deleted)

Tests are added and can be run using:
1. Run:
   $ ktd --shell
  k$ prove t/db_dependent/Koha/PseudonymizedTransaction.t \
           t/db_dependent/Koha/Pseudonymization.t
=> SUCCESS: New tests pass, old tests keep passing

Signed-off-by: AFHDubCoLib <andrewfh@dubcolib.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>