Add value and form properties to button component

[MaartenUreel]

https://unfoldadmin.com/docs/components/button/

The form and value attribute are very common, especially in Django:

• When the submit button is floating at the bottom, outside of the tag, you need to specify the name of the form to make the button work. So, to create custom pages that integrate visually we need to provide this attribute.
• When using e.g. django-formtools wizard, you can jump to another step by specifying the button value with the step ID.

I know we can provide this using attrs but then it cannot be defined purely in the front-end, we need to involve backend just to add a simple attribute which clutters the code. It would be nice if these were integrated in the component so we can use it more.

[lukasvinclav]

Awesome feature. Please draft a PR which will allow to create a dict directly in template. I believe jinja has already something like that. Example code:

{% create_dict "{'a': 'b', 'c': 'd'}" %}