Investigate use of systemd integration for user creation

Currently we rely on useradd and usermod from the shadow project for the creation of users in signstar-configure-build.

It should be investigated whether we can switch to a more declarative/ indirect approach, e.g. using sysusers.d or similar.

The prerequisite is to have users (but not their home directories!) set up in the read-only rootfs before the image is finalized. Currently we run signstar-configure-build in a mkosi postinst script.