arrow

GH-44569: [GLib] Add GArrowDecimal64DataType (#44571)



### Rationale for this change

`arrow::Decimal64Type` data type has been introduced. It is also necessary to support the same data type in GLib.

### What changes are included in this PR?

This PR implements `GArrowDecimal64DataType`.

### Are these changes tested?

YES

### Are there any user-facing changes?

Before this change: `Arrow::DecimalDataType.new(8, 2)` returned `Decimal128DataType`.
After this change: `Arrow::DecimalDataType.new(8, 2)` returns `Decimal64DataType`

* GitHub Issue: #44569

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