Consider adding a parameter to nest aggregated columns: aggregate(under: String?)

[koperagen]

df.groupBy { key }.aggregate {
  1 into "col"
}

schema:

key
col

df.groupBy { key }.aggregate(under = "statistics") {
  1 into "col"
}

schema:

key
statistics:
  col

Make sure it's consistent across other aggregate overloads (if makes sense)