feat!: Rename struct fields of rle output to len/value and update data type of len field

[stinodego]

Closes #15230

Changes

• The struct fields of the rle method have been renamed from lengths/values to len/value.
• The data type of the len field has been updated to match the index type (was previously Int32, now UInt32).

Example

Before

>>> s = pl.Series(["a", "a", "b", "c", "c", "c"])
>>> s.rle().struct.unnest()
shape: (3, 2)
┌─────────┬────────┐
│ lengths ┆ values │
│ ---     ┆ ---    │
│ i32     ┆ str    │
╞═════════╪════════╡
│ 2       ┆ a      │
│ 1       ┆ b      │
│ 3       ┆ c      │
└─────────┴────────┘

After

>>> s.rle().struct.unnest()
shape: (3, 2)
┌─────┬───────┐
│ len ┆ value │
│ --- ┆ ---   │
│ u32 ┆ str   │
╞═════╪═══════╡
│ 2   ┆ a     │
│ 1   ┆ b     │
│ 3   ┆ c     │
└─────┴───────┘

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.29%. Comparing base (e63e6fa) to head (695611c).

Files	Patch %	Lines
crates/polars-ops/src/series/ops/rle.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15249      +/-   ##
==========================================
- Coverage   81.30%   81.29%   -0.01%     
==========================================
  Files        1356     1356              
  Lines      175680   175681       +1     
  Branches     2522     2522              
==========================================
- Hits       142829   142827       -2     
- Misses      32368    32370       +2     
- Partials      483      484       +1     

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