Clarify expected cache headers

[fabricedesre]

From discord:

[10:46 AM] fabrice:
why is "no-cache" used for the default cache header value? 
[10:59 AM] Arqu:
Lots of those bits are still not fully fleshed out
[10:59 AM] Arqu:
Technically we cache everything if the store is present
[11:00 AM] Arqu:
But we dont handle the etags and cache headers fully so it doesnt make sense we advertise anything else
[11:00 AM] Arqu:
The blessing with content addressed storage is you can cache a cid indefinitely technically as the contents wont change
[11:01 AM] Arqu:
And you never cache ipns type reqs

I'm not sure if it's worth exposing cache headers as configurable. As @Arqu says, behavior needs to different between /ipfs and /ipns. For raw and car response, we should return Cache-Control: immutable.

Thoughts?

[fabricedesre]

The IPFS gateway spec section for the Cache-Control response header is at https://github.com/ipfs/specs/blob/main/http-gateways/PATH_GATEWAY.md#cache-control-response-header

There are no differences in https://github.com/ipfs/specs/blob/main/http-gateways/TRUSTLESS_GATEWAY.md

[Arqu]

Spec is already being followed, in the next update the misleading flags will be removed. The only exception is that for the IPNS based Last-Modified date we follow what kubo does instead of taking the spec recommendation to either do dns ttl based dates or not at all.