refactor(rust): Add Graphviz physical plan visualization for new streaming engine

[orlp]

This is fairly quick-and-dirty implementation, just set POLARS_VISUALIZE_PHYSICAL_PLAN="tmp.dot" before running your command to get a visualization. Going to be helpful while debugging the new streaming engine.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.

Project coverage is 80.19%. Comparing base (276990e) to head (a748c02).
Report is 28 commits behind head on main.

Files	Patch %	Lines
crates/polars-stream/src/physical_plan/mod.rs	0.00%	25 Missing ⚠️
crates/polars-stream/src/skeleton.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18307   +/-   ##
=======================================
  Coverage   80.19%   80.19%           
=======================================
  Files        1501     1501           
  Lines      198915   198944   +29     
  Branches     2837     2837           
=======================================
+ Hits       159521   159548   +27     
- Misses      38866    38868    +2     
  Partials      528      528           

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[ritchie46]

Nice! So set the env var with the location to write it to?

[orlp]

Nice! So set the env var with the location to write it to?

Yep :)