GH-44713: [Python] Add support for Decimal32 and Decimal64 types

[paleolimbot]

Rationale for this change

Arrow C++ and the Arrow specification now support 32-bit and 64-bit decimal types...pyarrow should too!

What changes are included in this PR?

Added type, array, and scalar bindings.

Are these changes tested?

Working on it!

Are there any user-facing changes?

Yes! (Working on completing documentation)

• GitHub Issue: [Python] Add support for Decimal32 and Decimal64 #44713

[github-actions]

⚠️ GitHub issue #44713 has been automatically assigned in GitHub to PR creator.

[ianmcook]

Please add the concrete classes for the scalars to the list under the Scalars heading in arrays.rst.

And after you add the is_ methods, please add those under the Type Checking heading in datatypes.rst.

Thanks!

[paleolimbot]

Current battles (so I remember them Monday!):

I'm missing some member definition for the use-facing type classes:

import pyarrow as pa

pa.decimal128(2, 0).precision
#> 2
pa.decimal256(2, 0).precision
#> 2
pa.decimal32(2, 0).precision
#> AttributeError: 'pyarrow.lib.Decimal32Type' object has no attribute 'precision'

There's no cast implemented from "float" to "decimal32/64", which at least some of the tests depend on:

pyarrow.lib.ArrowNotImplementedError: Unsupported cast to decimal32(7, -8) from float