feat(mkosi): Add minimal configuration for SignstarOS

Add signstar configuration that specifies only the default administrator
user, and those for up and downloading shared secrets and a dummy
connection to a local NetHSM user.

Signed-off-by: David Runge <dvzrv@archlinux.org>

resources/mkosi/signstar/mkosi.extra/usr/share/signstar/config.toml

+iteration = 1
+
+[[connections]]
+url = "https://localhost:8443/api/v1/"
+tls_security = "Unsafe"
+
+[[users]]
+nethsm_only_admin = "admin"
+
+[[users]]
+
+# NOTE: the authorized_keys are dummy entries for now
+[users.system_only_share_download]
+ssh_authorized_keys = ["ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOh96uFTnvX6P1ebbLxXFvy6sK7qFqlMHDOuJ0TmuXQQ user@host"]
+system_user = "ssh-share-down"
+
+[[users]]
+
+# NOTE: the authorized_keys are dummy entries for now
+[users.system_only_share_upload]
+ssh_authorized_keys = ["ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOh96uFTnvX6P1ebbLxXFvy6sK7qFqlMHDOuJ0TmuXQQ user@host"]
+system_user = "ssh-share-up"