Bug 23173: Provide core infrastructure

Bug 23173: Provide core infrastructure

This patch adds the required infrastructure to enable ILL availability
plugins to intercept the request creation process and, using the
supplied metadata, search for and display possible relevant items from
whichever availability plugins are installed.

Currently three availability plugins exist:

z39.50 - Searches any number of the Koha instance's configured Z targets
https://github.com/PTFS-Europe/koha-plugin-ill-avail-z3950

EDS - Searches the EBSCO Discovery Service
https://github.com/PTFS-Europe/koha-plugin-ill-avail-eds

Unpaywall - Searches the Unpaywall API for possible open access versions
of the requested item
https://github.com/PTFS-Europe/koha-plugin-ill-avail-unpaywall

The Unpaywall plugin is intended to serve as a "reference" plugin as the
API it deals with is extremely simple

Signed-off-by: Niamh Walker-Headon <Niamh.Walker-Headon@tudublin.ie>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>