GH-49055: [Ruby] Add support for writing decimal128/256 arrays (#49056)

### Rationale for this change

Decimal128/256 arrays are only supported.

### What changes are included in this PR?

Add `ArrowFormat::DecimalType#to_flatbuffers`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #49055

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