Trying to add a box shadow to the entire group within a grouped virtuoso

[scottwarren]

Trying to achieve something like:

https://codesandbox.io/s/spring-cdn-vvg3wd?file=/App.js

but the docs mention that the components that can be customised don't include a way of grouping the header and the items within that group

[scottwarren]

I think this is similar in scope to: #592