feat: Hold string cache in new streaming engine and fix row-encoding

[ritchie46]

#20947

[codecov]

Codecov Report

Attention: Patch coverage is 70.32967% with 27 lines in your changes missing coverage. Please review.

Project coverage is 79.27%. Comparing base (98ccb09) to head (5a01b75).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...es/polars-core/src/chunked_array/ops/row_encode.rs	68.85%	19 Missing ⚠️
crates/polars-core/src/series/mod.rs	58.82%	7 Missing ⚠️
crates/polars-expr/src/groups/row_encoded.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21039      +/-   ##
==========================================
+ Coverage   79.21%   79.27%   +0.06%     
==========================================
  Files        1583     1585       +2     
  Lines      225545   225611      +66     
  Branches     2586     2588       +2     
==========================================
+ Hits       178664   178857     +193     
+ Misses      46291    46164     -127     
  Partials      590      590              

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[github-actions]

The uncompressed lib size after this PR is 38.7352 MB.

[github-actions]

The uncompressed lib size after this PR is 38.7312 MB.

[github-actions]

The uncompressed lib size after this PR is 38.7216 MB.