Allow specifying overscanThreshold as prop

## Use case

My grids render rows that fetch their own data via RTK Query on mount. This works well with `enableVirtualization`, but the default `overscanThreshold` of `4` is a little too low in practice. When users scroll quickly, many unloaded rows come into view and they briefly display loading placeholders. I can afford to render more rows ahead of the viewport, which would trigger earlier fetches and eliminate most of this flicker.

## Proposed solution

Expose `overscanThreshold` as a `DataGrid` prop so it can be configured per grid. This would let developers balance performance and perceived smoothness based on their actual data-loading behaviour.