refactor(nixos): make ZFS configuration optional

- move ZFS and snapshot manager settings out of the global base
- group snapshot files under common/optional/zfs
- enable the ZFS module on existing ZFS hosts
- make the global tmpfs setting overridable

systems/jeeves/default.nix

@@ -15,6 +15,7 @@ in
     "${inputs.self}/common/optional/syncthing_base.nix"
     "${inputs.self}/common/optional/update.nix"
     "${inputs.self}/common/optional/zerotier.nix"
+    "${inputs.self}/common/optional/zfs"
     ./monitoring
     ./docker
     ./services