feat: add support to alter backfill parallelism for streaming jobs

[shanicky]

Introduce a new mechanism to control backfill parallelism for streaming jobs, allowing separate tuning of resource usage during backfill phases.

• Add AlterBackfillParallelism RPC and proto definitions in ddl_service.proto
• Extend CatalogWriter trait and frontend handler to support ALTER ... SET BACKFILL_PARALLELISM
• Implement server-side handling in Meta service and DdlController for backfill parallelism changes
• Add rescheduling logic in GlobalStreamManager and ScaleController to update backfill parallelism safely
• Validate unsupported cases (e.g., Iceberg tables) and support deferred execution mode
• Add integration test verifying backfill parallelism switches after backfill completion
• Extend SQL parser and keywords to recognize BACKFILL_PARALLELISM syntax

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What's changed and what's your intention?

Summary
This PR introduces support for altering the backfill parallelism of streaming jobs through the DDL gRPC API and the corresponding frontend handlers. It enables users to adjust the parallelism used during backfilling operations, including support for deferred execution and resetting the parallelism setting.

Details

• Added new RPC messages (AlterBackfillParallelismRequest and AlterBackfillParallelismResponse) and the AlterBackfillParallelism method to the DDL gRPC service definition.
• Extended the CatalogWriter trait and its implementation with an asynchronous method alter_backfill_parallelism to communicate with the meta service.
• Implemented a new handler function handle_alter_backfill_parallelism to process alter backfill parallelism requests, including extracting parallelism values and validating input.
• Integrated the new handler into the main dispatch logic, supporting the operation on tables, indexes, sinks, and sources, while returning a not implemented error for materialized views and disallowing the operation on iceberg engine tables.
• Added stubs for the new method in testing utilities (MockCatalogWriter) to facilitate unit testing.
• Began partial implementation of the meta service handler for backfill parallelism alterations.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note