Add `Cache-Control: private, no-store` headers where appropriate

[tashian]

The recommendation is to set Cache-Control: private, no-store on any endpoint with sensitive information. Because while you can protect the traffic with TLS, you also need to keep sensitive information out of a client's (unencrypted) HTTP cache. I'm not sure how relevant this is to the API context of step-ca though—I've never seen an HTTP client library that caches content. But I guess the point here is that a client could cache any content unless we tell it not to.