Refactor the `configure_workers_and_start.py` script used internally by Complement.

[matrixbot]

This issue has been migrated from #16650.

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Base: develop

This pull request is intended for commit-by-commit review.

Original commit schedule, with full messages:

1. Remove obsolete "app" from worker templates

2. Convert worker templates into dataclass

3. Use a lambda for the worker name rather than search and replace later

4. Collapse WORKERS_CONFIG by removing entries with defaults

5. Convert listener_resources and endpoint_patterns to Set[str]

6. Tweak comments

7. Add merge_into

8. Remove special logic for adding stream_writers: just make it part of the extra config template

9. Rename function to add_worker_to_instance_map given reduction of scope

10. Add sharding_allowed to the WorkerTemplate rather than having a separate function for that

11. Use merge_into when adding workers to the shared config

12. Promote mark_filepath to constant

13. Add a --generate-only option