refactor(rust): Add multiplexer node to new streaming engine

[orlp]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 127 lines in your changes missing coverage. Please review.

Project coverage is 80.24%. Comparing base (4157e92) to head (35cfbf9).
Report is 24 commits behind head on main.

Files	Patch %	Lines
crates/polars-stream/src/nodes/multiplexer.rs	0.00%	124 Missing ⚠️
crates/polars-stream/src/morsel.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18241      +/-   ##
==========================================
- Coverage   80.30%   80.24%   -0.06%     
==========================================
  Files        1498     1500       +2     
  Lines      198751   198871     +120     
  Branches     2833     2837       +4     
==========================================
- Hits       159598   159587      -11     
- Misses      38626    38757     +131     
  Partials      527      527              

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