[Lens] multi field top values supports drag drop

[ghudgins]

Describe the feature:
A follow-on from #95079, this feature implements the drag / drop support for "Combining" 2+ fields and was part of the original design.

Describe a specific use case for the feature:
When I am dragging a field that supports terms (number / text)
And Hovering an existing terms field (or using the keyboard navigational equivalent)
I need to see a new "Combine fields" drop target
So I can access this functionality in one "drop" without having to click into the existing terms aggregation, click add, find the field

Clip from original design

This issue was descoped from the MVP of #95079 on 9 september 2021 CC @MichaelMarcialis @mbondyra @stratoula

[elasticmachine]

Pinging @elastic/kibana-vis-editors (Team:VisEditors)

[flash1293]

Combine fields is implemented