Bug 30360: Add methods for dealing with the JSON payload

Bug 30360: Add methods for dealing with the JSON payload

This patch adds high-level methods to handle the JSON payload
(de)serialization as needed. This allows devs implementing background
jobs to abstract themselves from the internals of the jobs handling.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/BackgroundJob.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>