Support Lists with FormBuilderList

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Package/Plugin version

latest

### What you'd like to happen

I can't find anything on this, or if there is a way that I'm not seeing, but essentially just allow lists of items.

Something like
```dart
FormBuilderList (
name: name,
icon: Icons.add,
etc...
)
```

and
```dart
FormBuilderDropdownList (
name: name,
icon: Icons.add,
etc...
)
```
Where when you tap/click the icon you add another item to the list.

or maybe
```dart
FormBuilderList (
icon: Icons.add,
child: FormBuilderDropDown(
)
)
```
so it knows what type of form field to add when you click the add button. I think that option is better possibly.