gateway: migrate subdomain and dnslink code

[lidel]

Logic that parses Host header and translates it to content path is still in Kubo.

We need to migrate it as well, because the gateway binary for Saturn MUST also support subdomain (for dweb.link) and DNSLink gateways (for things like https://en.wikipedia-on-ipfs.org).

Ref.

• kubo
  • https://github.com/ipfs/kubo/blob/f54b2bcf6061219f7f481e8660e6707ae3bc3c38/cmd/ipfs/daemon.go#L841-L842
  • https://github.com/ipfs/kubo/blob/4e8271b1021f7d31e7b201640e6214a521f328e7/core/corehttp/hostname.go#L42-L43
• https://github.com/ipfs/specs/blob/main/http-gateways/SUBDOMAIN_GATEWAY.md
• https://github.com/ipfs/specs/blob/main/http-gateways/DNSLINK_GATEWAY.md

Other ToDos:

• Update CAR Gateway example to support subdomains on localhost

[lidel]

@hacdias this is lower priority than PoC examples (#141 and #142), but something we will need for the production binary for dweb.link.

Not sure how tricky this is, but these additional types of gateways are essentially wrappers executed before passing request to the path one.

I think this is a good patter/model to follow in our library.
See mux and childMux in this file.