Table: selected rows in nested tables

Currently, the selected row logic is limited to top-level rows; [this rowKeyGetter](https://github.com/grafana/grafana/blob/c55c726c8cae70eecb427aea4b89258cf1a5a4ed/packages/grafana-ui/src/components/Table/TableNG/utils.ts#L815-L817) doesn't quite do it, and even if it did, we would need to auto-expand any nested top-level rows which contain a selected nested row.