feat: Add rounding for Decimal type

[coastalwhite]

As part of #19630. This makes decimals already a bit more usable.

[codecov]

Codecov Report

Attention: Patch coverage is 63.20755% with 39 lines in your changes missing coverage. Please review.

Project coverage is 79.37%. Comparing base (8cb7839) to head (1961a85).
Report is 54 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-ops/src/series/ops/round.rs	63.20%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19760      +/-   ##
==========================================
- Coverage   79.55%   79.37%   -0.19%     
==========================================
  Files        1544     1550       +6     
  Lines      213240   214849    +1609     
  Branches     2441     2460      +19     
==========================================
+ Hits       169643   170531     +888     
- Misses      43048    43759     +711     
- Partials      549      559      +10     

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