[C++][Compute] Add a pivot function

[pitrou]

Describe the enhancement requested

To support pivot or ("pivot wider") functionality such as in Pandas, we could add a dedicated hash-aggregate function returning a StructArray.

We might also add a normal aggregate function (which would return a StructScalar).

Component(s)

C++

[pitrou]

Implementing this will be made easier by the "scatter" function proposed in #44393