Add support for `className` to `<AnchoredOverlay>`

[sampart]

At present, <AnchoredOverlay> can't be assigned classes. This means that styling can't be applied to the associated overlay.

In our case, we needed to adjust its z-index, for example.

[broccolinisoup]

Hello @sampart 👋 Thanks for raising this issue. We agree that AnchoredOverlay should have support for className as same as our other components do. You are welcome to push a PR (I always appreciate contributions 🙏 ). Otherwise it is in our backlog now and we will get to it. Thanks!

[sampart]

Thank you! I'm not expecting to get time on this one myself, but I'll keep it in mind.

[romanpozdnov]

Hey everyone!

In the PR above, the relevant changes have been made. Type definitions have been accounted for, and documentation has been updated.

Happy to help out!