Bug 30410: Add a way for plugins to expose tasks they implement

Bug 30410: Add a way for plugins to expose tasks they implement

This patch implements a mechanism for plugins to tell Koha they
implement background jobs, by returning a mapping

```
    task => class
```

* _task_ is a string that will be used when queueing new tasks, to let
  the workers know which class to instantiate to launch the job (a.k.a.
  process).

* _class_ is a string for a class name.

For this to  work, plugins need to include the 'namespace' attribute in
their metadata. If they don't, they will be skipped when listing the
valid _background jobs_.

After this, high-level methods for letting plugins (easily) enqueue
their tasks will be provided.

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

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>