Bug 32680: Add cover images plugin hook

Bug 32680: Add cover images plugin hook

This patch adds a plugin hook to inject cover images into the templates

Test plan:
1) Apply all patches
2) Go to https://github.com/PTFS-Europe/koha-plugin-addBDSCovers
3) In the releases section, download the .kpz file
4) Upload this in the plugins section and enable the plugin
5) In either the OPAC or staff client, search the catalog
6) The results should have cover art from BDS covers
7) Click on a result and the detail page should also have the cover art

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>