[C++] Add a pivot_longer node

[westonpace]

Describe the enhancement requested

Pivoting longer is an operation that is useful when a single row contains multiple observations or measurements. For example:

time	left_temp	right_temp
1	10	20
2	15	18

The columns left_temp and right_temp are really the combination of a "feature" (location) and a "measurement" (temp). We can pivot longer to extract these into their own columns:

time	location	temp
1	left	10
1	right	20
2	left	15
2	right	18

The addition of this capability should make it possible to fully bind tidyr's pivot_longer and pivot_wider (the latter can be expressed using a combination of project and aggregate). However, there is a considerable amount of convenience that could still be added and much of this is captured in the parent issue #34265

Component(s)

C++

[westonpace]

It appears that DuckDb has tackled the pivot feature at the same time (and their implementation is much more complete). They use unnest for pivot longer but it's not clear to me that we could do the same thing here. It relies on the ability to create a sort of list comprehension out of field references to convert field references to elements into a field reference to a list element (I'm not sure exactly what this syntax is called). It's quite impressive :)