Don't schedule an LDAP sync (ldap_sync_all) while it's still running

[a-gerhard]

Is your feature request related to a problem? Please describe. I have a longer-running synchronization process (more than 10 hours), and a new synchronization task is triggered every two hours. This slows down the other sync processes significantly, and may make the server to close existing connections.

Describe the solution you'd like Only schedule ldap_sync_all if there is no other ldap_sync_all task running. Even better would be to make this decision per source.

Describe alternatives you've considered

• Schedule them, but ldap_sync_all checks if it is running and returns instantly if so.
• Allow admins to configure the interval (either in global authentik settings, or per source)

[BeryJu]

While multiple syncs can still be scheduled, no multiple syncs will be run since #7263

In addition you can set this flag to disable celery beat on everything but one worker: https://github.com/goauthentik/authentik/blob/main/authentik/core/management/commands/worker.py#L20-L26