gateway: add support for car-* query parameters

[hacdias]

Option (A) from #603 (comment). Specifications: ipfs/specs#472.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.74%. Comparing base (0bbdc80) to head (70904d7).

❗ Current head 70904d7 differs from pull request most recent head 65cd401. Consider uploading reports for the commit 65cd401 to get more accurate results

@@                     Coverage Diff                      @@
##           gateway-content-location     #604      +/-   ##
============================================================
- Coverage                     59.79%   59.74%   -0.05%     
============================================================
  Files                           238      238              
  Lines                         29847    29860      +13     
============================================================
- Hits                          17847    17841       -6     
- Misses                        10388    10400      +12     
- Partials                       1612     1619       +7     

Files	Coverage Δ
gateway/handler.go	79.33% <100.00%> (+0.03%)	⬆️
gateway/handler_car.go	79.89% <100.00%> (+1.28%)	⬆️

... and 9 files with indirect coverage changes

[lidel]

Thanks, let's merge into #604.

There is one case where I want to add test (?format being different than Accept), but will do that there.