Bug - Modal - There's no way to pass a class to Backdrop

[rebeccaalpert]

Describe the problem
We're doing some things in virtual-assistant that require adjusting the Backdrop in the Modal. However, Modal does not let you directly pass classes to Backdrop. It only allows you to put classes on the dialog itself.

Expected behavior
There should be a prop that allows Modal to pass a className prop down through ModalContent to Backdrop.

Is this issue blocking you?
It is. I have to add a second Backdrop right now conditionally to get it to do what we need - I would prefer to drop it since it's confusing.

What is your product and what release date are you targeting?
virtual-assistant extension