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GH-44588: [GLib] Add GArrowDecimal64 Class (#44591)



### Rationale for this change

`Decimal64` class has been introeduced. It is necessary to support in GLib.

### What changes are included in this PR?

Implement `GArrowDecimal64` class.

### Are these changes tested?

YES

### Are there any user-facing changes?

NO

* GitHub Issue: #44588

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>