Bypass the conversion to/from intermediate format if not needed

[waterlx]

For Flink Iceberg sink, we used GenericRecord/Record as the intermediate format between input and writing to DataFile.
But this conversion is not needed when the input format is Avro IndexedRecord, and we are also writing to a DataFile with Avro format.
See this comment.
FileWriter#write() also needs to be type parameterized, see this comment.

[waterlx]

Seems need some help to include this issue to the milestone of "Flink Sink"

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[github-actions]

This issue has been closed because it has not received any activity in the last 14 days since being marked as 'stale'