[Lens] Pivot columns

[wylieconlon]

Pivot tables generally include grouped rows with subtotals and the most important feature, pivot columns.

Unlike pivot columns in spreadsheet tools, Lens can only display the table differently, not recalculate pivots as needed. There are two main cases:

2 buckets: Either bucket can be the pivot column, the other must be displayed as rows
3+ buckets: The pivot order is not flexible, but the user can choose whether to display 1 or 2 pivot columns. The last bucket is always displayed as rows, so the 1st and 2nd bucket can be pivoted.

A nice way of displaying this could be to include a section for "columns" and "rows" in the configuration panel on the right. The order of aggregations is still top-to-bottom like it is today:

Part of the tables meta issue and related to #61470

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)