fix: parse var len of decimal for parquet statistic

[ZENOTME]

I find that the decimal encode for parquet statistic will be byte with var len. Not necessary with 16 len. So we need to extend it when the len smaller than 16.

[ZENOTME]

cc @Xuanwo @liurenjie1024

[Xuanwo]

Hi, I assume the limitation of rust_decimal is that it only supports up to 28 digits of precision. Values of Decimal larger than Decimal::MAX or smaller than Decimal::MIN are not supported.

The best we can do here is raising an error if users trying to read or write Decimal larger than 28.

[ZENOTME]

But from the definition of iceberg spec, the precision of decimal can be 38. So I think for here we can't prevent user write decimal with precision larger than 28 but smaller than 38. https://iceberg.apache.org/spec/#primitive-types

[Xuanwo]

Yep, that is exactly the issue I raised here #669

[ZENOTME]

Actually, according to my understanding, our decimal is not restricted by rust_decimal.🤔 Internally iceberg have its own decimal representation.

For here, we just provide a way let the user pass a rust_decimal and convert it into our decimal representation. If the rust_decimal is that it only supports up to 28 digits of precision, when user want to more precision, they can pass other format to convert it into our decimal representation. Or as #669, we can provide a way to convert other decimal which support more precision to our decimal representation.