Schedule content for unpublishing X number of days after publishing

Patch Attached for Scheduler 2.x

I believe the above diff patch is missing and changes never get saved
+ $type->setThirdPartySetting('scheduler', 'unpublish_datetime', $form_state->getValue('scheduler_unpublish_datetime'));

also the issue described in #10 can occur with either NULL or '' so I did the following instead
+ if (!empty($unpublish_default)) {

New patch for 2.21 attached with those changes from #15

#15 was also missing node specific schema i think. updating from #16
+ unpublish_datetime:
+ type: string
+ label: 'Add a default for unpublishing'

To make progress here, it needs to be in a Merge Request so that pipeline testing is run.