[Bug]: [IcebergIO] Use RecordWriterManager for Partitioned writes

[ahmedabu98]

What happened?

We should use RecordWriterManager instead of just one RecordWriter to write rows grouped by partition. Doing just one RecordWriter can be dangerous for large writes where one partition has a very large amount of data. Currently, it will lead to one huge file, and the worker will appear idle to the runner the whole time it's writing.

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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