feat(python): Make expressions containing Python UDFs serializable

[stinodego]

Changes:

• Update Expr::AnonymousFunction to implement SerDe for Python functions.
• Update Expr::RenameAlias to raise an error when serializing instead of simply being skipped. Can implement SerDe for this later.
• Update the Polars Cloud prep functionality accordingly.

[codspeed-hq]

CodSpeed Performance Report

Merging #18135 will not alter performance

_{Comparing cloud-allow-lambda (2d9704f) with main (c03e760)}

Summary

✅ 37 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 63.82979% with 34 lines in your changes missing coverage. Please review.

Project coverage is 79.87%. Comparing base (c03e760) to head (2d9704f).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/expr_dyn_fn.rs	63.15%	21 Missing ⚠️
crates/polars-plan/src/dsl/python_udf.rs	63.88%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18135      +/-   ##
==========================================
+ Coverage   79.86%   79.87%   +0.01%     
==========================================
  Files        1501     1501              
  Lines      202029   202069      +40     
  Branches     2868     2868              
==========================================
+ Hits       161342   161400      +58     
+ Misses      40140    40122      -18     
  Partials      547      547              

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[ritchie46]

Looks great. Got some minor comments.

[ritchie46]

So we could add a PythonRenamAlias as well, I presume?

[stinodego]

Yes - we'd have to do something similar to PythonUdfExpression where we store the lambda as a PyObject.

But it's not super trivial and name.map and name.map_fields are implemented differently... I'd rather pick these up separately as I don't think those are too important and it will require some minor refactoring I think.