[Managed Iceberg] Make manifest file writes and commits more efficient

[ahmedabu98]

When writing to Iceberg, we need to write just one manifest file per snapshot.

However, we are currently writing one manifest file per bundle (or one per GIB batch for streaming writes), which is a lot more frequent than needed. In medium/large streaming jobs, we can end up with thousands of extra manifest files. For an Iceberg table, the effect of this inefficiency is felt in two ways:

• Writing more files than necessary
• During query planning, having to open and read more files than necessary

Solution:
Continue writing bundles/batches to data files, but stop writing manifest files at that cadence. Instead, group data files by destination then write and commit just one manifest file per destination. Essentially, the number of manifest files should be 1-1 with snapshots/commits (currently, it's roughly 1-1 with data files).

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[ahmedabu98]

Added as a release blocker because these are update-incompatible changes. Streaming writes are going to be officially supported in 2.60.0 so this should get in with it to avoid breaking pipeline update

[ahmedabu98]

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