rollup for freq_agg and topn_agg

[gugu]

Is your feature request related to a problem? Please describe.
I have several dailytopn_agg() values in materialized view and I want to aggregate them to get topn_agg() values for a week, for example

Describe the solution you'd like
Add rollup function for topn_agg() and freq_agg() the same way as it works for other aggregates

CREATE MATERIALIZED VIEW hourly AS
    SELECT timebucket('1hour', dt) as hour, TOPN_AGG(100, field) as field_topn FROM raw_table GROUP BY 1;
SELECT topn(10, rollup(field_topn)) FROM hourly WHERE hour BETWEEN 'A' and 'B'

Describe alternatives you've considered
It it possible to get all the topn_agg() results to the app and aggregate in business logic, but it requires downloading of 6MB hourly data to get a monthly value

Additional context
There is already space_saving_aggregate_combine rust function, which does exactly what I need, but it is not exposed