Flatten configuration/ qpdf expansion

// Flatten merges annotation appearances with page content, deleting the
// original annotations.
func (engine *QPdf) Flatten(ctx context.Context, logger *zap.Logger, inputPath string) error {
	var args []string
	args = append(args, inputPath)
	args = append(args, "--generate-appearances")
	args = append(args, "--flatten-annotations=all")
	args = append(args, "--replace-input")
	args = append(args, engine.globalArgs...)

	cmd, err := gotenberg.CommandContext(ctx, logger, engine.binPath, args...)
	if err != nil {
		return fmt.Errorf("create command: %w", err)
	}

	_, err = cmd.Exec()
	if err == nil {
		return nil
	}

	return fmt.Errorf("flatten PDFs with QPDF: %w", err)
}