Bug 20669: Add upgrade method to plugins

Bug 20669: Add upgrade method to plugins

It would be nice if plugins had an upgrade method to handle altering tables and such when a new version of a plugin is installed. Right now it must be done in a completely bespoke manner.

Test Plan:
1) Apply this patch
2) Download the 2 most recent releases of the Kitchen Sink plugin
   https://github.com/bywatersolutions/koha-plugin-kitchen-sink/releases
3) Install the earlier release
4) Look at the configuration page, at the bottom it should say the
   plugin has never been upgraded
5) Install the newer release
6) Reload the configuration page, it should now say that the plugin
   was upgraded and give you the date and time of the upgrade

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>