For some types of data it makes sense to collapse/expand some rows, not repeating the same value for this group and adding sub-summary rows for these groups

Kibana reference issue: elastic/kibana#61470

Open questions (this list is incomplete):

• Should this be possible in a nested way as shown in the screenshot or just feature a single level for collapsing/expanding?
• How is the functionality exposed? As rowspan/colspan and special styling plus everything else handled by the consumer or a more high level API?