[WIP][Form] Choice children can be template customized like collection

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | ?
| Deprecations? | no
| Tests pass?   | No ?
| Fixed tickets | 
| License       | MIT
| Doc PR        | None

I wanted to customize the template of a children of a choice field. I learned it was not currently possible, though it is with the collection type. So this should let people be able to customize the template for all the choice children at once.

Ie:

```jinja
{% block _user_colors_entry_widget %}
```

Still have to fix the tests