Implement arrow_json encoder for Decimal128 & Decimal256 DataTypes

[phillipleblanc]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

I would like to be able to use the arrow_json::ArrayWriter to write Arrow batches that contain Decimal128 & Decimal256 data.

Describe the solution you'd like

Running this code does not produce an error:

    #[test]
    fn test_decimal128_encoder() {
        let array = Decimal128Array::from_iter_values([1234, 5678, 9012])
            .with_precision_and_scale(10, 2)
            .unwrap();
        let field = Arc::new(Field::new("decimal", array.data_type().clone(), true));
        let schema = Schema::new(vec![field]);
        let batch = RecordBatch::try_new(Arc::new(schema), vec![Arc::new(array)]).unwrap();

        let mut buf = Vec::new();
        {
            let mut writer = LineDelimitedWriter::new(&mut buf);
            writer.write_batches(&[&batch]).unwrap();
        }

        assert_json_eq(
            &buf,
            r#"{"decimal":12.34}
{"decimal":56.78}
{"decimal":90.12}
"#,
        );
    }

Describe alternatives you've considered

N/A

Additional context

N/A

[alamb]

label_issue.py automatically added labels {'arrow'} from #6606