perf(rust): Move morsel distribution to the computational async engine

[orlp]

There was no reason we moved back to the I/O executor just for this.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.95%. Comparing base (72cd66a) to head (31f0599).
Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-stream/src/nodes/io_sources/parquet/init.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20600      +/-   ##
==========================================
- Coverage   78.95%   78.95%   -0.01%     
==========================================
  Files        1564     1564              
  Lines      220882   220882              
  Branches     2510     2510              
==========================================
- Hits       174407   174401       -6     
- Misses      45900    45906       +6     
  Partials      575      575              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.