feat(flow): avg func rewrite to sum/count

[discord9]

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What's changed and what's your intention?

added avg functino support

Please explain IN DETAIL what the changes are in this PR and why they are needed:

• added avg function support, which will be rewrite to sum(x)/count(x) if count(x)!=0
• fix aggr func's signature's output type to be the real evaled type(i.e. SumUInt* -> u64, SumInt* -> i64)

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 94.64720% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 85.37%. Comparing base (9f4a6c6) to head (426c742).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3955      +/-   ##
==========================================
- Coverage   85.64%   85.37%   -0.28%     
==========================================
  Files         973      973              
  Lines      167045   167411     +366     
==========================================
- Hits       143073   142931     -142     
- Misses      23972    24480     +508     

[fengjiachun]

LGTM