perf: Parallelize IO writes in new streaming engine

[coastalwhite]

Blocked on #20805

[ritchie46]

Block is merged

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 39 lines in your changes missing coverage. Please review.

Project coverage is 79.65%. Comparing base (5c261c0) to head (914e34f).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-stream/src/nodes/io_sinks/mod.rs	0.00%	37 Missing ⚠️
crates/polars-stream/src/nodes/io_sinks/csv.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20840      +/-   ##
==========================================
- Coverage   79.65%   79.65%   -0.01%     
==========================================
  Files        1570     1571       +1     
  Lines      223120   223171      +51     
  Branches     2546     2546              
==========================================
+ Hits       177736   177762      +26     
- Misses      44800    44825      +25     
  Partials      584      584              

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[coastalwhite]

Closing as this does not seem to be the bottleneck.