feat: implement avg and stddev for Int256 by float64

[shanicky]

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What's changed and what's your intention?

This PR attempts to add avg and stddev support for Int256. Because we currently only support conversion from Int256 to float64 (rather than decimal), we use float64 for these calculations.

Checklist For Contributors

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
  - [ ] I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• I have demonstrated that backward compatibility is not broken by breaking changes and created issues to track deprecated features to be removed in the future. (Please refer to the issue)
• All checks passed in ./risedev check (or alias, ./risedev c)

Checklist For Reviewers

• I have requested macro/micro-benchmarks as this PR can affect performance substantially, and the results are shown.

Documentation

Click here for Documentation

Types of user-facing changes

Please keep the types that apply to your changes, and remove the others.

• SQL commands, functions, and operators

Release note

[codecov]

Codecov Report

Merging #9217 (d7f1c9a) into main (ff8de5c) will decrease coverage by 0.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##             main    #9217      +/-   ##
==========================================
- Coverage   70.83%   70.83%   -0.01%     
==========================================
  Files        1201     1201              
  Lines      200454   200460       +6     
==========================================
- Hits       141992   141988       -4     
- Misses      58462    58472      +10     

Flag	Coverage Δ
rust	70.83% <77.77%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/expr/src/sig/agg.rs	18.69% <0.00%> (-0.36%)	⬇️
src/expr/src/sig/cast.rs	88.37% <100.00%> (+0.27%)	⬆️
src/expr/src/vector_op/arithmetic_op.rs	72.21% <100.00%> (+0.06%)	⬆️
src/expr/src/vector_op/cast.rs	89.07% <100.00%> (+0.01%)	⬆️
...rc/frontend/src/optimizer/plan_node/logical_agg.rs	96.28% <100.00%> (ø)

... and 9 files with indirect coverage changes

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