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

This issue has been migrated from [#16650](https://github.com/matrix-org/synapse/pull/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