[menu] Add `data-label` attribute to menu items

# Feature request

## Summary

It could be useful for different reasons to add a `data-label` attribute to the element that `Menu.Item` (or `Menu.CheckboxItem` or `Menu.RadioItem`) renders:

- Easily inspect how Base UI converts `children` into a string label (when `label` is not set), e.g. for debugging purposes.
- Write custom search/filtering functionality in the menu (this is what I'm trying to do).
- Maybe even style a specific menu item from an ancestor that doesn't control the rendering of individual items (though it would be a fragile selector).

## Examples in other libraries

N/A

## Motivation

See above. Thank you!