feat: Add JSON encoding for polars list type: pl.Expr.list.json_encode()

[tim-stephenson]

#14029
#8482

Involve most of the relevant discussion. It seems as though src/json/write/serialize.rs had all the relevant code, all that needed to be done was wrap these calls as the pl.Struct / struct_ does.

Is there any reason why all (or nearly all) polars types shouldn't have a .json_encode method? This might be preferred and reduce code duplication if it is desired to add a json_encode to more types than pl.Struct and pl.List.

A few lingering questions:

• Should we have a more universial json_encode to all (or nearly all) polars types
• Should inf & nan be treated differently to how the currently are? (see included test for current functionality)
• Should an option for mapping null rows to null rows with a ignore_nulls flag be present (instead of the current mapping to the string "null", similar to what feat: Add ignore_nulls option to struct.json_encode #17867 is working on?

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.85%. Comparing base (dd1fc86) to head (6ddb616).
Report is 11 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/list.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18353   +/-   ##
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  Coverage   79.84%   79.85%           
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  Files        1496     1496           
  Lines      200333   200357   +24     
  Branches     2841     2841           
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+ Hits       159952   159986   +34     
+ Misses      39856    39846   -10     
  Partials      525      525           

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[orlp]

Instead of adding this to the list namespace perhaps we should just consider making everything json_encode-able and putting it in the root namespace.

[tim-stephenson]

Instead of adding this to the list namespace perhaps we should just consider making everything json_encode-able and putting it in the root namespace.

I would agree withing making everything json_encode-able. My guess would be that all data types should be able to work besides polars.datatypes.Object.