Resolve ""Error getting performance bar results for [UUID]" regularly seen in production"

When a request contains an ETag value in its If-None-Match header, the backend may send a request ID (from Rack) that does not correspond to a value in Peek's Redis cache (because we aborted the Rails processing in our ETag middleware).

Because a cached response (304) has to replace the headers with those from the previous 200 - https://tools.ietf.org/html/rfc7234#section-4.3.4 - we add a custom header that will only be present in cache hits, that can tell the frontend to ignore these.

Closes #45406 (closed).