DNS-based Discovery: DNS Resolver

[jm-clius]

Subtask of #452
Created as partial fulfilment of #552

Problem

vacp2p/rfc#385 proposes a method to discover a bootstrap list of Waku v2 peers via DNS. This list is encoded in TXT records, stored in a domain. A DNS Resolver component must be created that can look up TXT records from a given subdomain.

NOTE: Since this component is easy to stub, it can be developed last.

[Menduist]

Hey, just stumbled into this issue.
FIY, we also have to resolve TXT dns as part of the dnsaddr resolving in libp2p, so for vacp2p/nim-libp2p#580 we added the dnsclient library, and integrated it in chronos.

Once this is merged (currently finishing up), you will be able to just do:
let record = await switch.nameResolver.resolveTxt("status.im") (or something like that)

We also plan to integrate other resolvers in the future (eg, Dns-Over-Https), which you could also benefit from using our resolver :)

[jm-clius]

Oh, dope! Thanks a lot, @Menduist. This will make my task significantly easier :)