Flink: Add extensibility support to IcebergSink for downstream composition

[herbherbherb]

Summary

• Add CommittableMetadata framework: a composition-based extension point allowing downstream to
  attach custom metadata to committables flowing through the sink pipeline
• Widen access modifiers on sink classes to enable downstream connector implementations to compose
  with (reference, wrap, delegate to) Iceberg's existing sink infrastructure

Resolves #15315.

Changes

New files (3, in both v2.0 and v2.1)

• CommittableMetadata.java — Serializable marker interface for custom metadata
• CommittableMetadataSerializer.java — serializer interface for metadata
• CommittableMetadataRegistry.java — global registry (downstream calls register() before
  sink creation)

Modified files (in both v2.0 and v2.1)

• IcebergCommittable — public class, public accessors, optional @Nullable metadata field,
  backward-compatible constructor chaining
• IcebergCommittableSerializer — writes boolean metadata flag + delegates serialization
• IcebergCommitter — public class + constructor
• IcebergSinkWriter — public class + constructor
• IcebergWriteAggregator — public class + constructor
• IcebergSinkBuilder — public interface
• IcebergSink — protected constructor, protected getters, protected Builder()
• CachingTableSupplier — public class + constructor
• RowDataTaskWriterFactory — protected getters for spec/format/outputFileFactory
• SinkUtil — public checkAndGetEqualityFieldIds()

Backward compatibility

• All existing constructors still work (new IcebergCommittable constructor chains with
  metadata = null)
• Serialization is backward-compatible (boolean flag distinguishes metadata presence)
• No behavioral changes to any existing code path

Test plan

• Existing TestIcebergCommitter passes
• Existing TestIcebergSink* tests pass
• Serialization backward compatibility: old serialized data (no metadata flag) still
  deserializes correctly

[bryanck]

The general approach is first make your change to the current version (2.1), then once that is merged, cherry pick to the other supported versions (separate PRs for each version).

[herbherbherb]

The general approach is first make your change to the current version (2.1), then once that is merged, cherry pick to the other supported versions (separate PRs for each version).

ack, thank you