discover: Add 'plugin' user-event

discover: Add 'plugin' user-event

Add a new user event to advertise pb-plugins and add them to the
device_handler. Plugins described by this event can either be
uninstalled pb-plugin files or successfully installed pb-plugins
depending on the associated parameters.
The is primarily intended for use by the pb-plugin utility itself to
notify Petitboot as it operates on pb-plugin files.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>