Add scheduler support

The code provided in the merge requests worked for me.

This patch was working well for us with Scheduler 2.0.*, but since updating to 2.1.0 we're every cron run failing to complete, and this error appearing in the log:

Error: Class "Drupal\scheduler\Event\SchedulerStorageEvents" not found in constant() (line 177 of /path/to/drupal/web/modules/contrib/scheduler/src/SchedulerManager.php)

I had a look at the changelog for the Scheduler 2.1.0 realease, and I don't see anything obvious:

https://www.drupal.org/project/scheduler/releases/2.1.0 →

For now I've made the problem go away by just removing scheduling from the storage type (I didn't have to uninstall Scheduler). I'll make some time to see what I can do to address this, but if anyone else wants to take a look it looks like the patch being developed for scheduling Paragraphs might point in the right direction:

https://www.drupal.org/project/scheduler/issues/3312200 ✨ Paragraph scheduler plugin Needs work

MR!9 didn't work for me. Cron is not running, publishing is still not happening at scheduled time. It still shows unpublished.
I've been waiting for this to be resolved for a long time, I was hoping it would be fixed with version 1.4 but the same problem persists. Thanks