Bug 35536: Add Koha object classes for plugin_data

Bug 35536: Add Koha object classes for plugin_data

Test plan:
Read the patch.
The objects will be used in subsequent patches, and tested there.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 5ef6a3fce70efe372733ac5c0a3ff267cd799838)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>