Add evals devservices mode

[JoshFerge]

Summary

• add an evals devservices mode for local seeded eval ingestion
• keep the mode focused on error-event evals: Relay, Snuba/Postgres, objectstore, taskbroker, ingest-events, post-process-forwarder-errors, and dedicated taskworkers for relay, ingest.errors, ingest.errors.postprocess, and issues
• avoid using the broader ingest mode or a catch-all taskworker for this local eval path

Why

Seeded Seer evals need enough of local Sentry ingestion to accept error events, save them, post-process them, and handle issue-side work such as auto source-code config. The existing ingest mode is broader than this path needs, while the generic taskworker can let slower issue-side work starve ingest.errors saves. This mode keeps that topology in Sentry devservices rather than in Seer helper scripts.

Verification

• git diff --check origin/master..HEAD
• .venv/bin/prek run -q check-yaml --files devservices/config.yml
• devservices config loader check for evals mode and supervisor dependency types
• pre-push hooks

[hubertdeng123]

did we expect this to have concurrency 2 compared to the others?

[JoshFerge]

yes, trying to make seeding local scenarios fast as possible. may add more depending on if machines can take it as well. 🙏🏼