feat: Add RoundMode for Decimal and Float

[Julian-J-S]

fix #21800 (though as @orlp mentioned there are theoretically more round modes to add)

Current Problem ⚠️

• current round expression has no mode and is inconsistent
  • Decimal: uses "half to even"
  • Float: uses "half away from zero"

PR Solution 🍀

• implement missing round modes
  • Decimal: implement "half away from zero"
  • Float: implement "half to even"
• set "half to even" as default for float & decimal

Example

df = pl.DataFrame(
    {
        "f64": [-3.5, -2.5, -1.5, -0.5, 0.5, 1.5, 2.5, 3.5],
        "d": ["-3.5", "-2.5", "-1.5", "-0.5", "0.5", "1.5", "2.5", "3.5"],
    },
    schema={
        "f64": pl.Float64,
        "d": pl.Decimal(scale=1),
    },
)

df.with_columns(
    pl.all().round().name.suffix("_default"),
    pl.all().round(mode="half_away_from_zero").name.suffix("_away_from_zero"),
    pl.all().round(mode="half_to_even").name.suffix("_to_even"),
)

shape: (8, 8)
┌──────┬──────────────┬─────────────┬──────────────┬────────────────────┬──────────────────┬─────────────┬──────────────┐
│ f64  ┆ d            ┆ f64_default ┆ d_default    ┆ f64_away_from_zero ┆ d_away_from_zero ┆ f64_to_even ┆ d_to_even    │
│ ---  ┆ ---          ┆ ---         ┆ ---          ┆ ---                ┆ ---              ┆ ---         ┆ ---          │
│ f64  ┆ decimal[*,1] ┆ f64         ┆ decimal[*,1] ┆ f64                ┆ decimal[*,1]     ┆ f64         ┆ decimal[*,1] │
╞══════╪══════════════╪═════════════╪══════════════╪════════════════════╪══════════════════╪═════════════╪══════════════╡
│ -3.5 ┆ -3.5         ┆ -4.0        ┆ -4.0         ┆ -4.0               ┆ -4.0             ┆ -4.0        ┆ -4.0         │
│ -2.5 ┆ -2.5         ┆ -2.0        ┆ -2.0         ┆ -3.0               ┆ -3.0             ┆ -2.0        ┆ -2.0         │
│ -1.5 ┆ -1.5         ┆ -2.0        ┆ -2.0         ┆ -2.0               ┆ -2.0             ┆ -2.0        ┆ -2.0         │
│ -0.5 ┆ -0.5         ┆ -0.0        ┆ 0.0          ┆ -1.0               ┆ -1.0             ┆ -0.0        ┆ 0.0          │
│ 0.5  ┆ 0.5          ┆ 0.0         ┆ 0.0          ┆ 1.0                ┆ 1.0              ┆ 0.0         ┆ 0.0          │
│ 1.5  ┆ 1.5          ┆ 2.0         ┆ 2.0          ┆ 2.0                ┆ 2.0              ┆ 2.0         ┆ 2.0          │
│ 2.5  ┆ 2.5          ┆ 2.0         ┆ 2.0          ┆ 3.0                ┆ 3.0              ┆ 2.0         ┆ 2.0          │
│ 3.5  ┆ 3.5          ┆ 4.0         ┆ 4.0          ┆ 4.0                ┆ 4.0              ┆ 4.0         ┆ 4.0          │
└──────┴──────────────┴─────────────┴──────────────┴────────────────────┴──────────────────┴─────────────┴──────────────┘

Open

• there are other round modes that can be implemented later

---

@orlp @wence-
sorry for creating another PR (old: #21883). I was overwhelmed and stuck in merge/rebase hell.
I implemented your requests. Please have a look 🤓

[codecov]

Codecov Report

Attention: Patch coverage is 56.64336% with 62 lines in your changes missing coverage. Please review.

Project coverage is 80.77%. Comparing base (69ad1ba) to head (f1832d4).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-ops/src/series/ops/round.rs	51.75%	55 Missing ⚠️
crates/polars-python/src/conversion/mod.rs	50.00%	5 Missing ⚠️
.../polars-python/src/lazyframe/visitor/expr_nodes.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22248      +/-   ##
==========================================
- Coverage   80.78%   80.77%   -0.01%     
==========================================
  Files        1639     1639              
  Lines      235548   235618      +70     
  Branches     2714     2714              
==========================================
+ Hits       190278   190322      +44     
- Misses      44630    44655      +25     
- Partials      640      641       +1     

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