refactor(iroh-net): allow to set a custom DNS resolver on the magic endpoint

[Frando]

Description

This makes the DNS resolver to be used in the context of a MagicEndpoint configurable. Up to now, we used a single, global, unconfigurable DNS resolver, stored in a per-process global static. This PR changes this so that we can set a DNS resolver in the builder of the MagicEndpoint. This resolver is passed through to all places where we need a DNS resolver - which is all places where we need to resolve relay URLs.

The default is unchanged: A single, shared DNS resolver is used for all endpoints. However, this default can now be changed per-endpoint. The global resolver is only used as a default in the endpoint builder if no custom resolver is set, and in the doctor, and in tests.

This change will make testing things that use DNS - prominently: #2045 - much easier. And we now have the means in place for people to customize the DNS resolving, if needed.

Notes & open questions

This makes the hickory_resolver::TokioAsyncResolver part of the public API surface of iroh-net.

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.