refactor(meta): extract independent ckpt job enum

[wenym1]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

This PR refactors meta checkpoint control by introducing a dedicated IndependentCheckpointJobControl abstraction for jobs that run on their own partial graphs, and migrating the existing creating-
streaming-job path onto that abstraction.

Concretely:

• Add checkpoint/independent_job as the new home for independent partial-graph checkpoint logic.
• Introduce IndependentCheckpointJobControl as the wrapper type for independently-checkpointed jobs.
• Move the existing creating-streaming-job control code under checkpoint/independent_job/creating_job.

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

[wenym1]

• feat(meta): support periodic refresh run for batch refresh job #25271
• feat(frontend): support consuming snapshot of batch refreshable job (frontend part) #25093
• feat(meta): support consuming snapshot of batch refreshable job (meta part) #25251
• feat(stream): allow snapshot backfill executor without upstream merge #25250
• refactor(meta): extract independent ckpt job enum #25249 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.