Add index to FormDataConsumer when used inside an ArrayInput

[dalen]

Is your feature request related to a problem? Please describe.

When writing some types of field level validators for fields inside an ArrayInput it would be useful to be able to get the index of the current component in some way. But I can also imagine other use cases for it.

Describe the solution you'd like

FormDataConsumer exposes formData & scopedFormData. I would like it to also expose index

Describe alternatives you've considered

Additional context

[djhi]

This is not properly documented but there is a useSimpleFormIteratorItem hook that returns the index as you can see in the <SimpleFormIterator> documentation.

However, this implies to create a component which FormDataConsumer was actually meant to avoid. Can you open a PR for this targeting the next branch?