feat: Support sub json path for JsonReader

[eranfu]

Users can use it to parse the partial data from the json. See the comment of with_sub_json_path for example.

[codecov]

Codecov Report

Attention: Patch coverage is 86.76471% with 9 lines in your changes missing coverage. Please review.

Project coverage is 79.80%. Comparing base (0654544) to head (999dba7).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-io/src/json/mod.rs	86.76%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21226      +/-   ##
==========================================
- Coverage   79.88%   79.80%   -0.09%     
==========================================
  Files        1593     1596       +3     
  Lines      227754   228488     +734     
  Branches     2600     2607       +7     
==========================================
+ Hits       181937   182337     +400     
- Misses      45220    45555     +335     
+ Partials      597      596       -1     

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[ritchie46]

This seems useful. However, this has to be exposed on the Lazy Readers and the Python API as well.