feat: add a builder for DnsResolver

[Frando]

Description

Based on #3473

In addition to the trait added in #3473 ,this adds a builder to allow setting more than one custom nameserver on the default hickory resolver, and to allow using other protocols than UDP.

The additional protocols are DNS over TCP, TLS, and HTTPS. For the latter two I enabled the https feature of hickory-resolver - this doesn't add any new dependencies, as tokio-rustls and h2 are in the tree already through reqwest.

Breaking Changes

Notes & open questions

There's two more protocols supported by hickory though behind non-default feature flags: Quic and H3. If there's demand, we could enable those too.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.
  • List all breaking changes in the above "Breaking Changes" section.
  • Open an issue or PR on any number0 repos that are affected by this breaking change. Give guidance on how the updates should be handled or do the actual updates themselves. The major ones are:
    • quic-rpc
    • iroh-gossip
    • iroh-blobs
    • dumbpipe
    • sendme

[github-actions]

Documentation for this PR has been generated and is available at: https://n0-computer.github.io/iroh/pr/3475/docs/iroh/

Last updated: 2025-09-30T09:13:25Z

[github-actions]

Netsim report & logs for this PR have been generated and is available at: LOGS
This report will remain available for 3 days.

Last updated for commit: 3b983cb

[dignifiedquire]

nice

[Frando]

I pushed a commit that has no functional changes, but makes it more obvious in the code that if we fail to load the host system's DNS config, we fallback to ResolverConfig::google. This was so far hidden in a unwrap_or_default (ResolverConfig::default calls ResolverConfig::google). I think we should have the fact that we fallback to google's DNS servers visible in the source code, so that if we want to change it to eg. cloudflare or something else, it's obvious to spot.