feat(batch): support decimal type for iceberg type

[chenzl25]

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What's changed and what's your intention?

• related issue: Feat: Batch ingest iceberg/file source #14742
• Support decimal type for iceberg source. We have used arrow-risingwave datatype conversion before, but since this conversion was designed for UDF originally it lacks some conversion between iceberg/arrow decimal and risingwave decimal, so we added the decimal support in this PR.

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[liurenjie1024]

Generally LGTM, left some comments.

[liurenjie1024]

LGTM, thanks!