Refactor storageapi conversion

[reuvenlax]

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Summary of Changes

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This pull request introduces robust support for automatic schema evolution in the BigQuery Storage Write API. It enables pipelines to handle schema changes dynamically by buffering records that encounter schema mismatches, patching the BigQuery table schema, and then retrying the buffered records. This change significantly improves the resilience of streaming pipelines writing to BigQuery.

Highlights

• Storage API Schema Evolution: Implemented support for automatic BigQuery table schema updates when using the Storage Write API, allowing for field additions and relaxations.
• Schema Buffering and Patching: Introduced new DoFns (ConvertMessagesDoFn, PatchTableSchemaDoFn, SchemaUpdateHoldingFn) to buffer records that fail schema validation until the table schema is successfully updated.
• Append Client Management: Refactored AppendClientCache to improve thread safety and resource management, including better handling of pinned clients and asynchronous closing.
• Schema Hash Verification: Added schema hashing to StorageApiWritePayload to detect when a client's view of the schema is out of date compared to the BigQuery table.

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