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GH-43956: [Format] Allow Decimal32/Decimal64 in format (#43976)

### 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.

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### 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: #43956

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Co-authored-by: Antoine Pitrou <pitrou@free.fr>