auth: generalise auth flow and introduce AuthVerdict

Generalise the registration pipeline to a more general auth pipeline
supporting both node registrations and SSH check auth requests.
Rename RegistrationID to AuthID, unexport AuthRequest fields, and
introduce AuthVerdict to unify the auth finish API.

Add the urlParam generic helper for extracting typed URL parameters
from chi routes, used by the new auth request handler.

Updates #1850

cmd/headscale/cli/debug.go

@@ -37,7 +37,7 @@ var createNodeCmd = &cobra.Command{
 		name, _ := cmd.Flags().GetString("name")
 		registrationID, _ := cmd.Flags().GetString("key")
 
-		_, err := types.RegistrationIDFromString(registrationID)
+		_, err := types.AuthIDFromString(registrationID)
 		if err != nil {
 			return fmt.Errorf("parsing machine key: %w", err)
 		}