combobox: Add a way to customize virtualizer options (e.g., gap, overscan, etc)

[ericbf]

Hello! Thanks for integrating the virtualizer. I was doing it manually before and was getting the phantom scrolling issues.

One problem is that with the embedded implementation, there is no ability to configure any of the other options to the virtualizer, it’s mostly hardcoded. Specifically we are needing to set gap between options, but it’s not exposed. Could we expose other virtualizer options in the virtual param of combobox?