Bug 18208: Add RecordProcessor filter to inject not onloan count to MARC records

Bug 18208: Add RecordProcessor filter to inject not onloan count to MARC records

This patch adds a new filter for MARC records to be used with
Koha::RecordProcessor. It's purpose is to inject the information about
items not-onloan in a fixed subfield, 999$x.

To test:
- Apply the patch
- Run:
  $ prove t/db_dependent/Koha/Filter/EmbedItemsAvailability.t
=> SUCCESS: Tests pass!
- Sign off :-D

Sponsored-by: ByWater Solutions
Followed test plan, test passes OK
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>