feat: JSON encoding in the root expression namespace pl.Expr.json_encode

[tim-stephenson]

Adding json_encode to the root expression & series namespace. This maps a column of any polars type (besides the object type) into a string column with it's corresponding JSON encoding.

Prior PR which only added it to the list namespace:

• feat: Add JSON encoding for polars list type: pl.Expr.list.json_encode()  #18353

Issues asking for this feature:

• Add json_encode to pl.List #14029
• Provide Expr.to_json() #8482

Current PR adding the ignore_nulls option to json_encode. I added (but did not yet implement) this option to the json_encode function I added.

• feat: Add ignore_nulls option to struct.json_encode #17867

[codecov]

Codecov Report

Attention: Patch coverage is 84.84848% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.80%. Comparing base (dd1fc86) to head (eaf6dee).
Report is 57 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/mod.rs	33.33%	2 Missing ⚠️
...ates/polars-plan/src/dsl/function_expr/dispatch.rs	92.30%	1 Missing ⚠️
.../polars-python/src/lazyframe/visitor/expr_nodes.rs	0.00%	1 Missing ⚠️
py-polars/polars/expr/struct.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18396      +/-   ##
==========================================
- Coverage   79.84%   79.80%   -0.04%     
==========================================
  Files        1496     1497       +1     
  Lines      200333   200407      +74     
  Branches     2841     2843       +2     
==========================================
- Hits       159952   159938      -14     
- Misses      39856    39944      +88     
  Partials      525      525              

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[ritchie46]

@tim-stephenson will this be finished? Otherwise we must close it because of the clutter.