Add tool for creating user integration when building OS image

With !75 (merged) we are adding a custom image-based OS, that creates user integration during build time for a read-only rootfs. While the current approach is merely for testing/ showcase purposes, we want a more robust and dedicated approach.

The user integration relies on several data points (e.g. username, role, SSH authorized key(s)), which should be provided in a system-wide available, machine-readable data format.

Based on this data a set of configuration tools needs to assess its correctness (during build time but also during boot - see #77) and create user integration for the system based on it:

• add user with home in /var/lib/signstar/home/, but without creating the dir
• add tmpfiles.d to auto-create the user's home (/var is a tpm2 encrypted partition that is created on first boot)
• add SSH integration, that sets the user's allowed command (based on its role) and adds an authorized key