Adds read transforms that consume planned Iceberg changelog tasks and turn them into Beam CDC rows.

ReadFromChangelogs reads task batches produced by ChangelogScanner:

• Unidirectional task batches are read and emitted directly as INSERT or DELETE rows
• Large bidirectional task batches are read with full rows, filtered by primary-key overlap range, and routed into keyed insert/delete streams for downstream update resolution.
  • Records outside the overlap range are safely emitted directly because they cannot match an opposing change.

LocalResolveDoFn reads and resolves small bidirectional task batches in memory.

CdcResolver centralizes the logic for reconciling deletes and inserts for the same primary key. It emits changed pairs as UPDATE_BEFORE / UPDATE_AFTER, and leaves unmatched rows as DELETE or INSERT. Duplicate no-op pairs with identical non-PK fields are dropped.

Part of #38831

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