arrow

GH-44619: [GLib] Add GArrowDecimal32Scalar (#44628)



### Rationale for this change

The `arrow::Decimal32Scalar` has been released.
GLib needs to implement `GArrowDecimal32Scalar`.

### What changes are included in this PR?

Implement `GArrowDecimal32Scalar`.

### Are these changes tested?

YES

### Are there any user-facing changes?

NO

* GitHub Issue: #44619

Lead-authored-by: Hiroyuki Sato <hiroysato@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>