Automatically add hidden field with form step number

Benefits of automatically added hidden field:

1. we doesn't depends on twig engine and template
2. we can do not provide flow into templates
3. reduce possible errors, because this required field would be rendered automatically. I had spent near 2 hours to debug why my form resets when 2nd step was submited.
4. we shouldn't override `setDefaultOptions` method in each form and `getFormOptions` in each flow, because `flow_step` option adds and sets automatically