vzdump: send notifications via new notification module

vzdump: send notifications via new notification module

... instead of using sendmail directly.

If the new 'notification-target' parameter is set,
we send the notification to this endpoint or group.
If 'mailto' is set, we add a temporary endpoint and a
temporary group containg both targets.

This commit also refactors the old 'sendmail' sub heavily:
  - Use template-based notification text instead of endless
    string concatenations
  - Removing the old plaintext/HTML table rendering in favor of
    the new template/property-based approach offered by the
    `proxmox-notify` crate.
  - Rename `sendmail` sub to `send_notification`
  - Breaking out some of the code into helper subs, hopefully
    reducing the spaghetti factor a bit

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>