refactor(rust): Spawn threads on our rayon pool in new-streaming

[orlp]

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[codecov]

Codecov Report

Attention: Patch coverage is 1.75439% with 56 lines in your changes missing coverage. Please review.

Project coverage is 79.13%. Comparing base (4fc5929) to head (60b52e6).
Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-stream/src/nodes/joins/equi_join.rs	0.00%	56 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21012      +/-   ##
==========================================
- Coverage   79.20%   79.13%   -0.07%     
==========================================
  Files        1583     1583              
  Lines      225105   225109       +4     
  Branches     2581     2581              
==========================================
- Hits       178294   178150     -144     
- Misses      46221    46369     +148     
  Partials      590      590              

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