Add support for custom portal container in floating components (Dialog, Dropdown, Popover, etc.)

[rezasohrabi]

Hi, First of all, thank you for your amazing work on Headless UI.

I'm working on a browser extension project where the app is rendered inside a shadow DOM.
In this context, it's important to have control over where portals are mounted.

Currently, libraries like shadcn/ui and Hero UI allow passing a custom container to their portal components (e.g., Dialogs, Dropdowns, Popovers, Listboxes).
However, Headless UI's floating components currently hardcode the portal target (typically document.body) without exposing a way to override it.
Since Headless UI is meant to be "headless", it would be great to allow developers to specify a custom container for portals.
This would make the library even more flexible and suitable for advanced use cases like Shadow DOM.
Thank you for considering this feature request!