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

[reivilibre]

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