refactor(rust): Groundwork for allowing multi-output nodes in the new streaming engine

[orlp]

This does not yet allow a node with multiple output schemas, that will come in a follow-up PR.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 156 lines in your changes missing coverage. Please review.

Project coverage is 78.97%. Comparing base (da0b589) to head (c947568).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...ates/polars-stream/src/physical_plan/lower_expr.rs	0.00%	73 Missing ⚠️
crates/polars-stream/src/physical_plan/mod.rs	0.00%	51 Missing ⚠️
crates/polars-stream/src/physical_plan/lower_ir.rs	0.00%	13 Missing ⚠️
crates/polars-stream/src/graph.rs	0.00%	9 Missing ⚠️
crates/polars-stream/src/execute.rs	0.00%	7 Missing ⚠️
crates/polars-stream/src/physical_plan/to_graph.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20550      +/-   ##
==========================================
- Coverage   79.00%   78.97%   -0.03%     
==========================================
  Files        1564     1564              
  Lines      220619   220704      +85     
  Branches     2502     2502              
==========================================
+ Hits       174300   174301       +1     
- Misses      45745    45829      +84     
  Partials      574      574              

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