Custom report column callbacks should be able to define column output according to the current aggregation method being applied to them

We can implement this by passing the current aggregation type as an argument to each callback (aggregation types supporting callbacks being avg/groupconcat/groupconcatdistinct/max/min/sum)

This restores some little-used functionality from Workplace, albeit implemented in a simpler way, and also solves most of the requirements from MDL-76199