Switch to hickory-resolver for DNS queries

[aumetra]

Is your feature request related to a problem? Please describe.

DNS queries are currently done using the standard system resolver, which means that for each time a domain name is resolved, we use a tokio blocking thread under the hood to invoke a system call.
This works well to some extent but at some point this can become a bottleneck, especially in ActivityPub environments, where there is a lot of different domain names that have to be resolved quite often.

Describe the solution you'd like

The hickory-dns project provides a user-land async Rust implementation of the DNS protocol.
This would allow us to keep all the resolve logic in-process. And due to it being async from the ground up, we don't need to spawn any blocking threads, getting rid of the expensive inter-thread communication and thread spawning.

It would also allow us to easier support:

1. DNS-over-TLS and DNS-over-HTTPS
2. Non-system DNS resolvers (for example, the system uses the default ISP-provided DNS servers, while Kitsune uses Quad9)