refactor(rust): separate Aggregation evaluation PhysicalExpr from conversion

[coastalwhite]

This PR separates the evaluation of the AggregationExpr PhysicalExpr to its own method. In the way, I removed several no-longer-used fields and parameters.

[codecov]

Codecov Report

Attention: Patch coverage is 96.73203% with 5 lines in your changes missing coverage. Please review.

Project coverage is 81.46%. Comparing base (15981d7) to head (6832925).
Report is 14 commits behind head on main.

Files	Patch %	Lines
crates/polars-expr/src/expressions/aggregation.rs	96.66%	4 Missing ⚠️
crates/polars-expr/src/planner.rs	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16688      +/-   ##
==========================================
- Coverage   81.47%   81.46%   -0.01%     
==========================================
  Files        1416     1416              
  Lines      186884   186843      -41     
  Branches     3021     3023       +2     
==========================================
- Hits       152257   152208      -49     
- Misses      34094    34103       +9     
+ Partials      533      532       -1     

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