feat: Support cumulative aggregations for Decimal dtype

[lukemanley]

Fixes #14745

import polars as pl
from decimal import Decimal as D

df = pl.Series("a", ["2.2", "1.1", "3.3"], pl.Decimal(21, 1)).to_frame()

df.select(
    pl.col("a").cum_sum().alias("cumulative_sum"),
    pl.col("a").cum_min().alias("cumulative_min"),
    pl.col("a").cum_max().alias("cumulative_max"),
)

BEFORE: (cum_sum raised InvalidOperationError):

┌────────────────┬────────────────┐
│ cumulative_min ┆ cumulative_max │
│ ---            ┆ ---            │
│ decimal[21,1]  ┆ decimal[21,1]  │
╞════════════════╪════════════════╡
│ 22.0           ┆ 22.0           │
│ 11.0           ┆ 22.0           │
│ 11.0           ┆ 33.0           │
└────────────────┴────────────────┘

AFTER:

┌────────────────┬────────────────┬────────────────┐
│ cumulative_sum ┆ cumulative_min ┆ cumulative_max │
│ ---            ┆ ---            ┆ ---            │
│ decimal[21,1]  ┆ decimal[21,1]  ┆ decimal[21,1]  │
╞════════════════╪════════════════╪════════════════╡
│ 2.2            ┆ 2.2            ┆ 2.2            │
│ 3.3            ┆ 1.1            ┆ 2.2            │
│ 6.6            ┆ 1.1            ┆ 3.3            │
└────────────────┴────────────────┴────────────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.78%. Comparing base (f0aa09b) to head (f9705e8).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20802   +/-   ##
=======================================
  Coverage   79.77%   79.78%           
=======================================
  Files        1561     1561           
  Lines      222029   222044   +15     
  Branches     2533     2533           
=======================================
+ Hits       177132   177152   +20     
+ Misses      44313    44308    -5     
  Partials      584      584           

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[ritchie46]

Nice one!