[Feature] Detecting cache hit

[yohannj]

Is your feature request related to a problem? Please describe.
I'd like to know if my query contacted ClickHouse or not, which allows me to know if I can trust the statistics which are in the ClickHouse response.

Describe the solution you'd like
Send a new header "X-Cache".
It could have two values: HIT, MISS
Or more values: HIT_LOCAL, HIT_SHARED, MISS

This non standard header is used by CDNs:
https://learn.microsoft.com/en-us/azure/cdn/cdn-msft-http-debug-headers
https://developer.fastly.com/reference/http/http-headers/X-Cache/
https://lyte.id.au/2014/08/28/x-cache-and-x-cache-lookupheaders/
https://aws.amazon.com/premiumsupport/knowledge-center/cloudfront-custom-object-caching/

Describe alternatives you've considered
chproxy returns a "Cache-Control: max-age=..." header. It can be used today if one knows the max-age of a query that has just been inserted in the cache, but for example a library has no way to know it, and it can get tricky in case of multiple cache being used with different duration.

Additional context