gateway: add Content-Location for non-default response formats

[hacdias]

Addresses https://github.com/orgs/ipshipyard/projects/1/views/1?pane=issue&itemId=58406852
Close #606

Specs PR: ipfs/specs#471

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.57%. Comparing base (2f8a18d) to head (f96974f).

@@            Coverage Diff             @@
##             main     #603      +/-   ##
==========================================
+ Coverage   59.72%   60.57%   +0.84%     
==========================================
  Files         238      238              
  Lines       29831    29462     -369     
==========================================
+ Hits        17817    17846      +29     
+ Misses      10405    10001     -404     
- Partials     1609     1615       +6     

Files	Coverage Δ
gateway/gateway.go	83.54% <ø> (ø)
gateway/handler.go	79.62% <100.00%> (+1.34%)	⬆️
gateway/handler_car.go	79.89% <100.00%> (+1.28%)	⬆️
gateway/hostname.go	74.36% <100.00%> (+0.17%)	⬆️

... and 42 files with indirect coverage changes

[hacdias]

@lidel before I open a PR for the specifications, we should decide what to do with CAR requests that contain "parameters" in the Accept header itself (e.g. dups, order).

These cannot be translated into query URL parameters, so the Content-Location header would not be fully correct. For now, I am not adding it. I wonder if we shouldn't instead make it possible to also set these parameters through the URL, or just keep is as it is.

[hacdias]

@lidel I've opened a PR in the specs repository: ipfs/specs#471

[lidel]

Thanks! I've reviewed ipfs/specs PRs, merged #604 into this one and pushed fix/test for case where format is different than Accept (91fdd90).

@hacdias If my changes look ok, feel free to merge, bubble up to rainbow, and bump our infra to rainbow image that includes this 🙏