ci: add gateway-conformance tests for car-gateway

[galargh]

This PR adds https://github.com/ipfs/gateway-conformance tests for CAR gateway example.

Note that subdomain-gateway spec is currently disabled in the gateway-conformance suite.

[codecov]

Codecov Report

Merging #237 (16cc265) into main (6f324be) will increase coverage by 0.29%.
The diff coverage is 63.63%.

❗ Current head 16cc265 differs from pull request most recent head 1c5519c. Consider uploading reports for the commit 1c5519c to get more accurate results

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   47.70%   48.00%   +0.29%     
==========================================
  Files         266      269       +3     
  Lines       32950    32986      +36     
==========================================
+ Hits        15718    15834     +116     
+ Misses      15549    15491      -58     
+ Partials     1683     1661      -22     

Impacted Files	Coverage Δ
examples/gateway/common/blocks.go	82.60% <63.63%> (-17.40%)	⬇️

... and 19 files with indirect coverage changes

[hacdias]

LGTM. I will be merging this. I'm investigating the subdomain in #246. It's probably similar to the issues we had in Bifrost (ipfs-inactive/bifrost-gateway#66), but I think there's something else too. Again, thanks @galargh @laurentsenta for your work on this!