Flink Sink V2: Add OutputFileFactoryProvider plugin interface

[herbherbherb]

Summary

Adds an OutputFileFactoryProvider plugin interface to IcebergSink that allows external implementations to customize how OutputFileFactory instances are created. This enables use cases like custom file naming, path rewriting, or alternative storage routing without subclassing internal writer factories.

Resolves #15763.

Changes

• New: OutputFileFactoryProvider.java -- @FunctionalInterface with a single method: OutputFileFactory create(Table, int taskId, int attemptId, FileFormat, PartitionSpec)
• Modified: RowDataTaskWriterFactory -- accepts optional provider, uses it in initialize() when present
• Modified: IcebergSink.Builder -- new outputFileFactoryProvider() method, passed through to writer factory

Compatibility

• No behavioral change when the provider is not set (null default)
• No changes to public API signatures of existing methods
• Fully backward compatible