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GH-44589: [GLib] Add GArrowDecimal32 class (#44597)



### Rationale for this change

The `Decimal32` class has been introduced. It is necessary to support it in GLib.

### What changes are included in this PR?

Implement `GArrowDecimal32` class

### Are these changes tested?

YES

### Are there any user-facing changes?

NO

* GitHub Issue: #44589

Authored-by: Hiroyuki Sato <hiroysato@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>