[EuiBasicTable] Should allow visual grouping of cells, especially columns

[wylieconlon]

This is not a request for full grouping support like the data grid will support, but for a simpler form of this: #1689

The default way that Kibana flattens aggregated data is to repeat all the values in a column, which results in a table with redundancy:

Is there a way to hide duplicate row values and present a more condensed version of the table? Maybe something like this?

[github-actions]

👋 Hey there. This issue hasn't had any activity for 180 days. We'll automatically close it if that trend continues for another week. If you feel this issue is still valid and needs attention please let us know with a comment.

[wylieconlon]

@chandlerprall this stale issues bot is probably wrong for big features like this one. We do still need this.

[wylieconlon]

Although we no longer need this in the BasicTable, we now need this in the DataGrid. Which does have this issue: #4635

[daveyholler]

Although we no longer need this in the BasicTable, we now need this in the DataGrid. Which does have this issue: #4635