Add `signstar-config` crate to handle Signstar configs, administrative and non-administrative credentials on a Signstar host

Add signstar-config and accompanying integration tests.

signstar-config/src/error.rs

@@ -133,9 +133,10 @@ pub enum ErrorExitCode {
     UtilsExecutableNotFound,
     UtilsSystemUserLookup,
     UtilsMappingSystemUserGet,
-    UtilsSystemUserNotRoot,
     UtilsSystemUserData,
     UtilsSystemUserMismatch,
+    UtilsSystemUserNotRoot,
+    UtilsSystemUserRoot,
     WriteToStdin,
 }
 
@@ -254,6 +255,7 @@ impl From<Error> for ErrorExitCode {
                     current_user: _,
                 } => Self::UtilsSystemUserMismatch,
                 crate::utils::Error::SystemUserNotRoot { user: _ } => Self::UtilsSystemUserNotRoot,
+                crate::utils::Error::SystemUserRoot { user: _ } => Self::UtilsSystemUserRoot,
             },
             // top-level errors and their exit codes
             Error::ApplyPermissions {