feat: Allow more group_by agg expressions in the new streaming engine

[orlp]

This should allow for elementwise combinations of streaming aggregates over elementwise combinations of the input columns.

[codecov]

Codecov Report

Attention: Patch coverage is 1.37931% with 286 lines in your changes missing coverage. Please review.

Project coverage is 78.86%. Comparing base (c4b704b) to head (35287be).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../polars-stream/src/physical_plan/lower_group_by.rs	0.00%	271 Missing ⚠️
...ates/polars-stream/src/physical_plan/lower_expr.rs	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20663      +/-   ##
==========================================
- Coverage   78.96%   78.86%   -0.10%     
==========================================
  Files        1557     1558       +1     
  Lines      220743   221023     +280     
  Branches     2527     2527              
==========================================
+ Hits       174318   174319       +1     
- Misses      45847    46126     +279     
  Partials      578      578              

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