Title: jwt_authn: JWKs health

Description:

Make it so that the state of the JWKs cache is reflected in the application (readiness) health.

Basically tie in readiness check to the JWKs cache, which should refresh if JWKs are missing or expired. This should result in

• JWKs are prefetched on startup, before traffic is accepted.
• suspended traffic if the cache expires and cannot be refreshed