ci(gateway): enable subdomain conformance

[hacdias]

This PR enables the subdomain conformance gateway tests (added in #237). Since we are using the CAR example to run the Gateway tests, we need to make it more conformant, of course, similarly to what happened in ipfs/bifrost-gateway#66.

• Enables subdomain conformance tests on example.net (domain used in examples)
• Adds CONNECT support to the gateway examples.
• Extract the handler creation login into the common subpackage since it was the same for both examples.

[codecov]

Codecov Report

Merging #246 (0bc8590) into main (b01359c) will increase coverage by 0.28%.
The diff coverage is 90.62%.

❗ Current head 0bc8590 differs from pull request most recent head 725a8f4. Consider uploading reports for the commit 725a8f4 to get more accurate results

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
+ Coverage   47.87%   48.15%   +0.28%     
==========================================
  Files         269      269              
  Lines       32976    32957      -19     
==========================================
+ Hits        15787    15871      +84     
+ Misses      15514    15422      -92     
+ Partials     1675     1664      -11     

Impacted Files	Coverage Δ
examples/gateway/car/main.go	14.63% <0.00%> (+6.63%)	⬆️
examples/gateway/proxy/main.go	0.00% <0.00%> (ø)
examples/gateway/common/http.go	93.54% <93.54%> (ø)

... and 12 files with indirect coverage changes

[lidel]

Thank you! 🚢

(while looking at this, found unrelated nit: #176 (review))