feat: migrate subdomain and dnslink code

[hacdias]

Closes #149.

Some things I want to improve:

• Do not require a name system. Instead, require some function or simple interface dedicated only to solve DNSLink domains. We don't need the full thing.
• Simplify the configuration and leave part of the configuration magic for Kubo.
• Remove /api and /p2p from hostname and keep in Kubo (related to point before)
• Working example
• Fix the cause of failing Kubo Go tests (refactor: migrate DNSLink and subdomain gateway code to go-libipfs kubo#9624)
  • I'm think it might be caused by us now calling ResolvePath instead of namesys.Resolve. This is because the namesys.Resolve is part of ResolvePath and ResolvePath does more than just namesys.Resolve. So we catch some weird errors here. Unlikely. ResolvePath returns the error of namesys.Resolve.
  • See https://app.circleci.com/pipelines/github/ipfs/kubo/9010/workflows/15e0dfe2-0ee6-4e86-8056-c9cb7d65521c/jobs/97462
  • See refactor: migrate DNSLink and subdomain gateway code to go-libipfs kubo#9624 (comment)
• Restore commented test (will require some work but will also give opportunity to move other some Go gateway tests); or move back to Kubo for now (may involve copying a few helper functions).
• PR in Kubo (refactor: migrate DNSLink and subdomain gateway code to go-libipfs kubo#9624)

Note: sharness expected to fail as we changed the Gateway API interface.

[codecov]

Codecov Report

Merging #153 (aa8a482) into main (a4e0934) will increase coverage by 1.11%.
The diff coverage is 45.90%.

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
+ Coverage   18.66%   19.77%   +1.11%     
==========================================
  Files          99      100       +1     
  Lines       10540    10968     +428     
==========================================
+ Hits         1967     2169     +202     
- Misses       8294     8512     +218     
- Partials      279      287       +8     

Impacted Files	Coverage Δ
examples/gateway/car/main.go	9.83% <0.00%> (-4.80%)	⬇️
examples/gateway/common/blocks.go	38.19% <0.00%> (-2.25%)	⬇️
examples/gateway/proxy/main.go	0.00% <0.00%> (ø)
gateway/gateway.go	0.00% <ø> (ø)
gateway/hostname.go	53.15% <53.15%> (ø)

[hacdias]

@lidel I updated the PR and the Kubo PR: ipfs/kubo#9624. Every test is passing now. I'm not very happy how I ended up handling it: I added a new function to the interface. Read more about the issue here: ipfs/kubo#9624 (comment) - I feel like the alternative would require me to do deeper changes to the Hostname code.

[lidel]

Thank you @hacdias!

Pushed small doc changes because extracted things lacked context outside Kubo.

Good enough for now, but we will refine the way configuration happens in the future.
(after bifrost-gateway / Kubo 0.19)