feat(CdkTable): There should be a way to access CdkTable's _renderRows

[ahmed-dardery]

Feature Description

The property _renderRow, which is a private property on CdkTable represent the currently rendered rows in the table. This is very convenient for accessing information about what's visible, which is some times more convenient than querying the dataSource.

Suggestion is something like:
private _renderRows: RenderRow<T>[]; get renderedRows() { return _renderRows }
Could also return a readonly version.

Use Case

Interested in having a "expand all / collapse all" button in the header of the table, which ideally should only loop over the rows that are currently visible to access their .data.expanded value. After which it can be changed to expand all or collapse all.