Add more system/other notification events

Notification events are a nice way to collect terse history of what the server is actually doing, but some more information would be nice...

Would like to see some more system notification events:

• server paused/resumed
• server started/shutting down

Would like to see plan/job events:

• plan build started/stopped
• job build started/stopped

I'm mostly interested in plan started, since some plans could take many many hours, I'd like to get a simple notification when this happens.

Would also like to see other notification events:

• elastic agent instance started/stopped/failed
• elastic agent running/stopped/failed

Might also be interesting to make a generic notification task, so that a particular job workflow could emit its own custom notifications.