notifications: allow sending notifications via proxmox_notify

notifications: allow sending notifications via proxmox_notify

  - Set the context in proxmox_notify
  - Add helper function which queues notifications to a spool
    directory
  - Set up a worker task, running in the privileged process, which
    periodically checks the spool directory for queued notifications

The queuing is needed because on PBS we send most if not all
notifications from the proxy-process running as the `backup` user.
However, to have access to the protected passwords/tokens for various
notification endpoints, we need to read the notification config as
root.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
Tested-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>