feat: Add Restart Service workflow

[taylor-osler-sentry]

Summary

Adds a new manually-triggered "Restart Service" GitHub Actions workflow that redeploys the currently deployed Cloud Run service revision, without rebuilding or changing any images.

How it works

• Triggered via workflow_dispatch with an environment input (test/prod), same as Deploy.
• No build step — versions are already pinned by image SHA in Artifact Registry.
• For each Cloud Run service (matching the same service matrices as deploy.yml):
  1. Authenticates via the same Workload Identity Federation setup as Deploy.
  2. Exports the currently deployed service definition with gcloud run services describe --format export.
  3. Re-applies that exact definition via google-github-actions/deploy-cloudrun (metadata input), forcing a new revision with identical config — effectively a restart.

Using the exported YAML avoids hardcoding multi-container flags (e.g. for firetower/firetower-test with the nginx sidecar), since the full existing spec is captured automatically.

[cursor]

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[spalmurray]

You could probably deduplicate the shared code between prod and test, but looks good otherwise!

[linear-code]

RELENG-945