feat: Store only the last row in the previous cursor for round robin tie-breaking purposes

[ariel-miculas]

Which issue does this PR close?

• Closes fix: reduce peak memory usage when round robin tiebreaker is disabled #23606.

Rationale for this change

See the linked issue.
Note that this PR also contains the changes in #23606, so this will have to be rebased

What changes are included in this PR?

Store only the last row in prev_cursors instead of keeping the entire cursor

Are these changes tested?

Added a test to show the improvement

Are there any user-facing changes?

No

[EmilyMatt]

I feel like this should return an Option, in case values() is empty

[ariel-miculas]

values() shouldn't be empty, there are some asserts in the code:

        assert!(rows.num_rows() > 0);
        assert!(array.len() > 0, "Empty array passed to FieldCursor");

I don't imagine a Cursor with empty values being useful.

impl RowValues {
    /// Create a new [`RowValues`] from `rows` and a `reservation`
    /// that tracks its memory. There must be at least one row
    ///
    /// Panics if the reservation is not for exactly `rows.size()`
    /// bytes or if `rows` is empty.
    pub fn new(rows: Arc<Rows>, reservation: MemoryReservation) -> Self {
        assert_eq!(
            rows.size(),
            reservation.size(),
            "memory reservation mismatch"
        );
        assert!(rows.num_rows() > 0);
        Self {
            rows,
            _reservation: reservation,
        }
    }
}

    pub fn new<A: CursorArray<Values = T>>(
        options: SortOptions,
        array: &A,
        reservation: MemoryReservation,
    ) -> Self {
        assert!(array.len() > 0, "Empty array passed to FieldCursor");
        let null_threshold = match options.nulls_first {
            true => array.null_count(),
            false => array.len() - array.null_count(),
        };

[ariel-miculas]

The previous code also assumed values is never empty:

    fn is_eq_to_prev_row_in_prev_batch(&self, other: &Self) -> bool {
        assert_eq!(self.offset, 0);
        T::eq(
            &self.values,
            self.offset,
            &other.values,
            other.values.len() - 1,
        )
    }