REST-Controlled Scheduled Tasks

Background
Currently, there's a gap in terms of how NX3 backups can be orchestrated. We want admins to export the database (i.e. via the scheduled task), and keep the export files safe, and synchronize this with a blob store/rest of Nexus backup. This is tricky because the scheduled task is controlled inside Nexus, while blob stores are often backed up using an externally controlled rsync.

Acceptance

• There is a mechanism to list configured scheduled tasks, query their run status and invoke them (as if they were being manually invoked).

Technical Notes

• Perhaps the right way to do this is as part of beefing up the Provisioning API to cover scheduled tasks (including finding and triggering them).
• We'll defer configuration-specific endpoints (and this can currently be done with the provisioning API).
• Consider that some tasks may not come from our own plugins.