feat(flow): flow runtime render reduce operator

[discord9]

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What's changed and what's your intention?

Add functionalitiy to render reduce operator, i.e. simple aggregate functions
Also refactor render.rs into smaller files:

• impl render_reduce which support distinct and basic aggr functions
• split render.rs into smaller files

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 93.00080% with 87 lines in your changes are missing coverage. Please review.

Project coverage is 85.19%. Comparing base (20e8c3d) to head (f377499).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3736      +/-   ##
==========================================
- Coverage   85.46%   85.19%   -0.27%     
==========================================
  Files         966      968       +2     
  Lines      162211   163167     +956     
==========================================
+ Hits       138630   139008     +378     
- Misses      23581    24159     +578