feat: Add `signstar-common` crate to provide common facilities
- Adds all public data required for the
signstar-configure-buildcrate, as it will be shared by other crates. - Adds NetHSM specific defaults.
- Adds basic configuration file retrieval and directory handling for Signstar hosts.
- Adds administrative credentials file retrieval and directory handling for Signstar hosts.
Signed-off-by: David Runge dvzrv@archlinux.org