gateway: extract CORS to headers middleware

[hacdias]

To help with ipshipyard/waterworks-community#4. Sharness won't pass here because Kubo won't build.

Kubo PR: ipfs/kubo#10311

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4c3a1f2) 65.58% compared to head (253640e) 65.61%.

❗ Current head 253640e differs from pull request most recent head 19948a2. Consider uploading reports for the commit 19948a2 to get more accurate results

@@            Coverage Diff             @@
##             main     #569      +/-   ##
==========================================
+ Coverage   65.58%   65.61%   +0.02%     
==========================================
  Files         206      207       +1     
  Lines       25587    25589       +2     
==========================================
+ Hits        16781    16789       +8     
+ Misses       7335     7330       -5     
+ Partials     1471     1470       -1     

Files	Coverage Δ
examples/gateway/common/handler.go	95.40% <100.00%> (-0.16%)	⬇️
gateway/gateway.go	75.94% <ø> (-9.10%)	⬇️
gateway/handler.go	76.46% <ø> (-0.25%)	⬇️
gateway/headers.go	100.00% <100.00%> (ø)
gateway/hostname.go	73.02% <33.33%> (-0.38%)	⬇️

... and 10 files with indirect coverage changes