Support complete distinct usage for aggregate expressions

[WinkerDu]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Here are multiple aggregate expressions have not support distinct usage yet.
Though optimizer SingleDistinctToGroupBy (#1315) supports single distinct usage, there are more SQL scenes have not been covered, like

SELECT AVG(c1), SUM(DISTINCT c2) FROM test

I would like to take effort to work on the following tickets to complete distinct usage for aggregate expressions

• sum(distinct) support #2405
• Introduce sum_distinct() function to dataframe #2407
• avg(distinct) support #2408
• Introduce avg_distinct() function to dataframe #2409
• var(distinct) support #2410
• median(distinct) support #2411
  (more to be added later)

Describe the solution you'd like
Adds specific aggregate expression support, like expressions::DistinctSum for sum(distinct)

Describe alternatives you've considered
No.

Additional context
No.

[WinkerDu]

@andygrove @alamb can you please assign this issue to me? Thank you ❤️

[alamb]

@andygrove @alamb can you please assign this issue to me? Thank you ❤️

Done -- thanks @WinkerDu