Support decimal for Iceberg source

[fuyufjh]

Is your feature request related to a problem? Please describe.

Iceberg supports the standard fixed-point decimal-128, which supports up to 38 base-10 digits i.e. Decimal(38, S). While RisingWave supports float-point decimal-128, as a cost, it's up to 28 base-10 digits (29 in some cases).

Describe the solution you'd like

In real cases, although the column type is defines with Decimal(38, S), most values won’t have 38 digits.

That why I think we can round the number and record a warning somewhere in case that a row was really rounded.

Describe alternatives you've considered

No response

Additional context

No response

[chenzl25]

After a discussion, we decided to let the iceberg source throw an error if it meets a decimal with precision that exceeds the precision risingwave can represent.