[CHORE] implement mean and stddev for decimal

[samster25]

closes: #3157

[codspeed-hq]

CodSpeed Performance Report

Merging #3159 will degrade performances by 16.11%

_{Comparing sammy/decimal-aggs (f903caf) with main (6e28b3f)}

Summary

⚡ 1 improvements
❌ 1 regressions
✅ 15 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	sammy/decimal-aggs	Change
⚡	test_iter_rows_first_row[100 Small Files]	306.5 ms	262.4 ms	+16.77%
❌	test_show[100 Small Files]	50.1 ms	59.8 ms	-16.11%

[codecov]

Codecov Report

Attention: Patch coverage is 67.19577% with 62 lines in your changes missing coverage. Please review.

Project coverage is 79.10%. Comparing base (6e28b3f) to head (f903caf).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/daft-dsl/src/functions/agg/mod.rs	57.53%	31 Missing ⚠️
src/daft-core/src/datatypes/agg_ops.rs	62.85%	13 Missing ⚠️
src/daft-core/src/datatypes/infer_datatype.rs	47.36%	10 Missing ⚠️
src/daft-core/src/series/ops/agg.rs	73.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3159      +/-   ##
==========================================
- Coverage   79.12%   79.10%   -0.03%     
==========================================
  Files         641      642       +1     
  Lines       78151    78294     +143     
==========================================
+ Hits        61837    61931      +94     
- Misses      16314    16363      +49     

Files with missing lines	Coverage Δ
src/daft-core/src/array/iterator.rs	100.00% <ø> (ø)
src/daft-core/src/array/ops/mean.rs	100.00% <100.00%> (ø)
src/daft-core/src/datatypes/mod.rs	40.00% <ø> (ø)
src/daft-dsl/src/expr/mod.rs	78.07% <100.00%> (+0.13%)	⬆️
src/daft-dsl/src/functions/mod.rs	84.21% <ø> (ø)
src/daft-functions/src/list/mean.rs	61.76% <100.00%> (ø)
...ft-physical-plan/src/physical_planner/translate.rs	95.59% <100.00%> (+0.01%)	⬆️
src/daft-core/src/series/ops/agg.rs	77.14% <73.33%> (-0.50%)	⬇️
src/daft-core/src/datatypes/infer_datatype.rs	81.53% <47.36%> (ø)
src/daft-core/src/datatypes/agg_ops.rs	72.91% <62.85%> (-27.09%)	⬇️
... and 1 more

... and 4 files with indirect coverage changes