refactor: Turn pl.cumulative_eval into its own AExpr

[coastalwhite]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 72.35772% with 34 lines in your changes missing coverage. Please review.

Project coverage is 80.42%. Comparing base (fb9a4e8) to head (3b4eb7c).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/expressions/eval.rs	74.02%	20 Missing ⚠️
crates/polars-plan/src/dsl/expr/mod.rs	38.46%	8 Missing ⚠️
crates/polars-plan/src/dsl/format.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #22994   +/-   ##
=======================================
  Coverage   80.41%   80.42%           
=======================================
  Files        1682     1680    -2     
  Lines      223305   223330   +25     
  Branches     2802     2802           
=======================================
+ Hits       179572   179612   +40     
+ Misses      43071    43056   -15     
  Partials      662      662           

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