[Iceberg]Fix identity and truncate transform on DecimalType column

[hantangwangd]

Description

When we create partitions on DecimalType columns(using identity or truncate transform) in Iceberg, we will meet various problems because of the inappropriate handling for decimal data.

This PR fix the problems appears when using decimal as a partition column.

Test Plan

• Test identity transform on column of ShortDecimalType type
• Test truncate transform on column of ShortDecimalType type
• Test identity transform on column of LongDecimalType type
• Test truncate transform on column of LongDecimalType type

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Release Notes

== RELEASE NOTES ==

Iceberg Changes
* Fix identity and truncate transforms on DecimalType columns

[steveburnett]

Instead of NO RELEASE NOTE, perhaps a release note entry similar to the following would be appropriate?

== RELEASE NOTES ==

Iceberg Changes
* Fix identity and truncate transforms on DecimalType columns

[hantangwangd]

@steveburnett Thanks for your reminder. Release note has been added!

[tdcmeehan]

Thanks for this fix. A couple of questions.

[tdcmeehan]

Does this preserve the precision?

[hantangwangd]

It didn't, and the precision would always be 0 in the result value of BigDecimal. But precision of 0 in BigDecimal is ok as it is not a strictly required parameter as scale. It could be gotten by function precision() if necessary.

[tdcmeehan]

Perhaps add a preconditions check to ensure we don't return just the unscaled value of a LongDecimalType?

[hantangwangd]

Sure, good suggestion. Fixed!

[tdcmeehan]

One last nit!

[hantangwangd]

@tdcmeehan Fixed. Please take a look, thanks!