Refresh Iceberg partition specs periodically

[AtharvUrunkar]

Summary

Fixes #38337

This change updates AssignDestinationsAndPartitions to periodically refresh cached Iceberg partition specs instead of loading them only once per worker instance.

Previously, PartitionKey and BeamRowWrapper instances were cached indefinitely, which could result in stale partition specs being used after table spec updates.

Changes

• Added periodic refresh logic using a refresh interval
• Added refresh timestamp tracking per table
• Reloaded partition specs after refresh interval expiration
• Preserved existing caching behavior between refreshes
• Added explicit non-null validation for cached objects before usage

Testing

• ./gradlew spotlessApply
• ./gradlew :sdks:java:io:iceberg:test

Note: Local Iceberg test execution on Windows encountered unrelated environment/runtime issues (URISyntaxException / Unsafe initialization errors) likely tied to Windows + Java toolchain compatibility rather than this isolated change.

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R: @kennknowles for label java.

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