refactor: Implement CSV, IPC and NDJson in the MultiScanExec node

[coastalwhite]

This PR adds support for CSV, IPC and NDJson in the MultiScanExec node and, overall the makes the node way more effective and efficient. This is still all locked behind POLARS_NEW_MULTIFILE=1 and should not change normal behavior.

[codecov]

Codecov Report

Attention: Patch coverage is 12.25000% with 351 lines in your changes missing coverage. Please review.

Project coverage is 78.97%. Comparing base (92fd75d) to head (7d85e6a).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-mem-engine/src/executors/multi_file_scan.rs	0.00%	147 Missing ⚠️
...tes/polars-mem-engine/src/executors/scan/ndjson.rs	16.92%	54 Missing ⚠️
crates/polars-mem-engine/src/executors/scan/ipc.rs	0.00%	53 Missing ⚠️
crates/polars-mem-engine/src/executors/scan/csv.rs	0.00%	42 Missing ⚠️
...es/polars-mem-engine/src/executors/scan/parquet.rs	11.11%	40 Missing ⚠️
crates/polars-mem-engine/src/planner/lp.rs	70.96%	9 Missing ⚠️
crates/polars-stream/src/nodes/io_sources/ipc.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20648      +/-   ##
==========================================
- Coverage   79.00%   78.97%   -0.04%     
==========================================
  Files        1566     1557       -9     
  Lines      221034   220690     -344     
  Branches     2510     2514       +4     
==========================================
- Hits       174618   174280     -338     
+ Misses      45842    45837       -5     
+ Partials      574      573       -1     

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