feat: Add new Int128Type

[coastalwhite]

This PR properly adds an Int128 type so that the Decimal type has a valid physical type.

[codecov]

Codecov Report

Attention: Patch coverage is 74.11972% with 147 lines in your changes missing coverage. Please review.

Project coverage is 79.59%. Comparing base (1cd807f) to head (eac30ed).
Report is 126 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-compute/src/cast/mod.rs	7.14%	26 Missing ⚠️
crates/polars-core/src/datatypes/any_value.rs	18.18%	18 Missing ⚠️
...es/polars-core/src/chunked_array/ops/row_encode.rs	75.51%	12 Missing ⚠️
crates/polars-core/src/series/mod.rs	85.71%	12 Missing ⚠️
crates/polars-row/src/encode.rs	86.36%	9 Missing ⚠️
...polars-core/src/series/implementations/duration.rs	0.00%	6 Missing ⚠️
...tes/polars-core/src/series/implementations/time.rs	0.00%	6 Missing ⚠️
...tes/polars-core/src/series/implementations/null.rs	0.00%	5 Missing ⚠️
crates/polars-row/src/decode.rs	78.26%	5 Missing ⚠️
crates/polars-arrow/src/io/ipc/write/schema.rs	0.00%	4 Missing ⚠️
... and 19 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20232      +/-   ##
==========================================
- Coverage   79.63%   79.59%   -0.04%     
==========================================
  Files        1565     1565              
  Lines      218192   218462     +270     
  Branches     2475     2475              
==========================================
+ Hits       173749   173879     +130     
- Misses      43876    44016     +140     
  Partials      567      567              

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[alexander-beedie]

Very nice!