[FEATURE] Support compound expression in Stats aggregation

[LantaoJin]

Is your feature request related to a problem?
Query stats max(age) by country works but stats max(age) / 2 by country doesn't.
Q8 in tpch:

	sum(case
		when nation = 'BRAZIL' then volume
		else 0
	end) / sum(volume) as mkt_share

could be straightforwardly rewritten to

stats sum(case(nation = 'BRAZIL', volume else 0)) / sum(volume) as mkt_share by o_year

But we haven't support compound expression in aggregation. So as a workaround, we have to change it to:

| stats sum(case(nation = 'BRAZIL', volume else 0)) as sum_case, sum(volume) as sum_volume by o_year
| eval mkt_share = sum_case / sum_volume
| fields mkt_share, o_year

What solution would you like?
A clear and concise description of what you want to happen.

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?
Add any other context or screenshots about the feature request here.