GH-43956: [Format] Allow Decimal32/Decimal64 in format

[zeroshade]

Rationale for this change

Widening the Decimal128/256 type to allow for bitwidths of 32 and 64 allows for more interoperability with other libraries and utilities which already support these types. This provides even more opportunities for zero-copy interactions between things such as libcudf and various databases.

What changes are included in this PR?

Updating the documentation in Schema.fbs to explicitly state that 32-bit and 64-bit is now allowed for bitwidths of Decimal types. This is the only area in the the spec that mentions the allowed decimal bitwidths.

• GitHub Issue: [Format] Add Decimal32 and Decimal64 to Arrow #43956

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit c4b68ee.

There were 4 benchmark results indicating a performance regression:

• Commit Run on amd64-m5-4xlarge-linux at 2024-09-11 01:22:10Z
  • file-write (R) with compression=uncompressed, dataset=nyctaxi_2010-01, file_type=parquet, input_type=dataframe, language=R
  • dataset-serialize (Python) with dataset=nyctaxi_multi_ipc_s3, format=parquet, selectivity=1pc
• and 2 more (see the report linked below)

The full Conbench report has more details. It also includes information about 119 possible false positives for unstable benchmarks that are known to sometimes produce them.