nixos: systemd-user: allow additional upstream user units
nixos: systemd-user: allow additional upstream user units

Currently it is only possible to add upstream _system_ units. The option
systemd.additionalUpstreamSystemUnits can be used for this.

However, this was not yet possible for systemd.user. In a similar
fashion this was added to systemd-user.nix.

This is intended to have other modules add upstream units.

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