refactor: Introduce Row{Encode,Decode} as FunctionExpr

[coastalwhite]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 89.16256% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.44%. Comparing base (b512da3) to head (6cf1976).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/mod.rs	0.00%	7 Missing ⚠️
...s/polars-plan/src/plans/aexpr/function_expr/mod.rs	41.66%	7 Missing ⚠️
py-polars/polars/expr/expr.py	60.00%	3 Missing and 1 partial ⚠️
.../polars-python/src/lazyframe/visitor/expr_nodes.rs	0.00%	2 Missing ⚠️
py-polars/polars/functions/lazy.py	83.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #23933    +/-   ##
========================================
  Coverage   81.44%   81.44%            
========================================
  Files        1672     1673     +1     
  Lines      225183   225330   +147     
  Branches     2903     2907     +4     
========================================
+ Hits       183394   183521   +127     
- Misses      41064    41082    +18     
- Partials      725      727     +2     

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[ritchie46]

Maybe we can expose it as an expression on structs as well. We have all the logic. I think it can be useful to more people.

As long as we don't guarantee a stable row-encoding across versions.