<RFC> Add a DNS over HTTPS backend resolver

[steamraven]

Some projects do not want to or cannot use the standard DNS backend to resolve multaddrs. For example, wasm in the browser does not have access to UDP DNS. Other projects may want extra security or do not want the underlying network to rewrite DNS queries

This commit adds a very simple DNS over HTTPS backend resolver for multiaddrs. Since it is using the JSON specification as implemented by Google and Cloudflare, the resulting resolver is super simple.

This could be broken out into a completely separate project (go-multiaddr-dns-doh)

Please let me know if I can improve this

[Stebalien]

I agree supporting DOH would be useful, but this code doesn't look multiaddr specific in any way. I assume someone using DOH would set the multiaddr dns resolver in their code and import their favorite DOH client library (possibly this one).

[lidel]

Quick feedback: We should avoid JSON requests. JSON representation is not an official DoH spec.

It may be standardized at one point, but right now, only the binary format defined by RFC8484 is future-proof.
To guarantee people can use resolvers other than Google and Cloudflare we should make queries in the binary format.

I wrote some notes in https://github.com/ipfs/js-ipfs/issues/2212, including test endpoints compatible with RFC8484, including one used by Firefox. Hope this helps if you wish to make a vendor-agnostic DoH (RFC8484) resolver.

[aschmahmann]

Just to clarify there are 2 action items here:

1. This PR is not spec compliant as @lidel mentioned above
2. This PR is a go-dns-doh resolver having nothing to do with multiaddr, you shouldn't be losing out on anything by just making this a separate repo as it could just be plugged into DefaultResolver here by application code.

[lidel]

Superseded by #26 – @aschmahmann are you able to close this?

[aschmahmann]

@lidel #26 isn't really related here other than adding more configurability to the resolver.

As I mentioned above plugging a DoH resolver into the interfaces should do the job. There is now a spec compliant DoH resolver at https://github.com/libp2p/go-doh-resolver.