SpatialNavigationVirtualizedList default position

**Is your feature request related to a problem? Please describe.**
Currently there is no way to set default/initial focus index on virtualized list
There is no even any ScrollTo method exposed

**Describe the solution you'd like**
Add prop for initial index, or expose anything like ScrollTo method

**Describe alternatives you've considered**
Wrapping stuff in

Start at the SpatialNavigationVirtualizedList entry point and inspect how focus and virtualized scrolling are currently handled. Clarify whether the intended result is an initial focus index, an exposed scroll method, or both; done should include a documented API and verified behavior for the requested default position.

Written by the indexing model from the issue text.