Advertise cache retention duration for cloudflare

[poszu]

motivation

We use Cloudflare in front of poets to cache some endpoints like /info, /proofs, /pow_params etc. Currently, cache retention is configured on the CF side, but it would be best if the poet notified CF for how long it can cache responses. This is possible in HTTP with the Cache-Control HTTP header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control.

It is possible to inject HTTP headers in responses in a GRPC-gateway (GRPC-HTTP proxy that we use) with https://grpc-ecosystem.github.io/grpc-gateway/docs/mapping/customizing_your_gateway/#mutate-response-messages-or-set-response-headers

endpoints

/info

It contains dynamic content (open and executing round IDs) that changes every round (epoch). It could advertise max-age till the end of round

/pow_params

It serves 2 values:

• challenge - it currently changes after a round is finished (it's the root of the last proof),
• difficulty - set in configuration, it can change after poet service was restarted.

/proofs

The content is static and never changes - can be cached indefinitely.

/submit

Should never be cached.