feat(operator): support configurable curl user-agent for SSI sync

[gautamkrishnar]

Summary

• Adds operator.sync.user-agent config property to the operator
• When set, propagates the value to the sidecar via two channels:
  • SIDECAR_SYNC_USER_AGENT key in the sidecar's ConfigMap (persistent across pod restarts)
  • userAgent field injected into the sync JSON payload sent to /api/sync
• When unset, no changes to existing behavior

Related

• Sidecar PR: feat(sidecar): support configurable curl user-agent for sync sidecar#24

Test plan

• Set operator.sync.user-agent=MyBot/1.0 and trigger a sync — verify SIDECAR_SYNC_USER_AGENT appears in the sidecar ConfigMap
• Verify Akamai/origin sees the custom user-agent during sync crawl
• Without the property set, verify sync behaves as before