An SSH private key (as an absolute file path or Systemd credential name), usually generated by rad auth.
If set to the default value of null, radicle will import the private key from a credential named xyz.radicle.node.secret.
If configured as a credential name it will be imported via ImportCredential= in the service configuration. Refer to the systemd-creds documentation for more details https://systemd.io/CREDENTIALS/