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

---------

Co-authored-by: Antoine Pitrou <[email protected]>

docs/source/format/Columnar.rst

@@ -21,7 +21,7 @@
 Arrow Columnar Format
 *********************
 
-*Version: 1.4*
+*Version: 1.5*
 
 .. seealso:: :ref:`Additions to the Arrow columnar format since version 1.0.0
    <post-1-0-0-format-versions>`

docs/source/format/Versioning.rst

@@ -105,3 +105,8 @@ Version 1.4
 * Added :ref:`listview-layout` and the associated ListView and LargeListView
   types.
 * Added :ref:`variadic-buffers`.
+
+Version 1.5
+-----------
+
+* Expanded Decimal type bit widths to allow 32-bit and 64-bit types.

format/Schema.fbs

@@ -24,6 +24,7 @@
 /// Version 1.3 - Add Run-End Encoded.
 /// Version 1.4 - Add BinaryView, Utf8View, variadicBufferCounts, ListView, and
 /// LargeListView.
+/// Version 1.5 - Add 32-bit and 64-bit as allowed bit widths for Decimal
 
 namespace org.apache.arrow.flatbuf;
 
@@ -222,17 +223,17 @@ table RunEndEncoded {
 }
 
 /// Exact decimal value represented as an integer value in two's
-/// complement. Currently only 128-bit (16-byte) and 256-bit (32-byte) integers
-/// are used. The representation uses the endianness indicated
-/// in the Schema.
+/// complement. Currently 32-bit (4-byte), 64-bit (8-byte), 
+/// 128-bit (16-byte) and 256-bit (32-byte) integers are used.
+/// The representation uses the endianness indicated in the Schema.
 table Decimal {
   /// Total number of decimal digits
   precision: int;
 
   /// Number of digits after the decimal point "."
   scale: int;
 
-  /// Number of bits per value. The only accepted widths are 128 and 256.
+  /// Number of bits per value. The accepted widths are 32, 64, 128 and 256.
   /// We use bitWidth for consistency with Int::bitWidth.
   bitWidth: int = 128;
 }