[2.0.0-beta1] Expose default attributes prop in forms?

We don't do it automatically because some (most?) projects doesn't want the attributes to be exposed for every components.

It is better to do case by case, by adding attributes in the props.

However, if a project wants this attributes and:

• has no possibility to edit the component
• doesn't wnat to edit all components

we need to provide a hook in the definitionsAlter of the compoennt plugin manager

Merged with ✨ [2.0.0-alpha1] Resolve Twig namespaces in _template property Active