refactor(rust): Reduction -> GroupedReduction for the new streaming engine

[orlp]

This is necessary preparation for the groupby aggregations in the new streaming engine, it will share the aggregation code with the grouped reductions.

[codecov]

Codecov Report

Attention: Patch coverage is 3.98818% with 650 lines in your changes missing coverage. Please review.

Project coverage is 79.64%. Comparing base (9dada18) to head (0c688b2).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/reduce/min_max.rs	0.00%	236 Missing ⚠️
crates/polars-expr/src/reduce/mod.rs	0.00%	168 Missing ⚠️
crates/polars-expr/src/reduce/sum.rs	0.00%	86 Missing ⚠️
crates/polars-expr/src/reduce/mean.rs	0.00%	79 Missing ⚠️
crates/polars-expr/src/reduce/len.rs	0.00%	33 Missing ⚠️
crates/polars-stream/src/nodes/reduce.rs	0.00%	21 Missing ⚠️
crates/polars-arrow/src/bitmap/mutable.rs	54.54%	15 Missing ⚠️
crates/polars-expr/src/reduce/convert.rs	0.00%	7 Missing ⚠️
crates/polars-utils/src/float.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19176      +/-   ##
==========================================
- Coverage   79.78%   79.64%   -0.14%     
==========================================
  Files        1531     1531              
  Lines      208445   208916     +471     
  Branches     2913     2418     -495     
==========================================
+ Hits       166301   166393      +92     
- Misses      41593    41976     +383     
+ Partials      551      547       -4     

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