Relates to b/147.
First step towards giving depot modules the ability to declare their
own backup directories by moving all restic configuration into a new
module and adding a NixOS option for inclusion/exclusion paths for
backups.
This still keeps all backup paths within the whitby config.
Change-Id: Ia96833668f1a3d02da892261153d8b02156b8ac0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3565
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
