[Iceberg CDC sink] table setup

[ahmedabu98]

Adds per-destination setup:

• TableSetup resolves and validates each destination (equality columns, schema against the table, format version, the partitioning rule), auto-creates missing tables, and pins the partition spec for the run.
• PartitionShardPlan derives write shards from the partition tuple under a shards_per_partition cap.
• CommitToken is the snapshot-summary contract the committer uses for exactly-once recovery and spec pinning.

Needs #39997 to get merged first.

Part of #39979

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