fix(gateway): Cache-Control header for UnixFS directories on /ipfs

[lidel]

Continuing from #639

• original fix from from fix(gateway): Cache-Control header for UnixFS directories #639
• limit it to immutable /ipfs to not over-cache DNSLink websites with default resolver (due to TTL gap described in Gateway: Cache-Control, max-age=ttl, ETag for /ipns requests #329 (comment)
• update tests
• CI is green

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.87%. Comparing base (0190b70) to head (1e23bb2).

@@            Coverage Diff             @@
##             main     #643      +/-   ##
==========================================
+ Coverage   59.75%   59.87%   +0.12%     
==========================================
  Files         238      238              
  Lines       29999    30004       +5     
==========================================
+ Hits        17926    17965      +39     
+ Misses      10450    10426      -24     
+ Partials     1623     1613      -10     

Files	Coverage Δ
gateway/handler_unixfs_dir.go	65.88% <100.00%> (+1.03%)	⬆️

... and 11 files with indirect coverage changes

[lidel]

Lgtm, we will likely refine cache-control across all response types, but this is useful fix of a hot path (dir listings on /ipfs should benefit from wider cache window).

Related spec clarification in ipfs/specs#480

Digression:

• max-age based on TTL for /ipns/{dnslink} is unrelated to this fix, but we may want to fix it sooner than later, as Gateway: Cache-Control, max-age=ttl, ETag for /ipns requests #329 (comment) impacts directory listings, and that is why this PR is limited to dir listings on /ipfs.
• We are not married to the stale-while-revalidate value on IPNS, could adjust if needed (mutable conformance tests only care about max-age), but 1 month sounds more robust than a few days or a week, the intention is to avoid link rot, if http cache supports such wide window.