Release v2.39.0

Added

• A service can opt into sticky routing with Backend.StickyRouting. AuthProxy discovers the backend's replicas through DNS (for example a Kubernetes headless Service) and keeps each browser on the same replica with an encrypted per-service cookie. It moves the user to another replica when the pinned one goes away or turns unhealthy. The feature is off by default, and existing services route exactly as before. (part of #209)