feat(iroh-dns-server)!: add dht fallback option (#2188)

## Description

feat(iroh-dns-server): add dht fallback option

if we don't have a record, ask the DHT as a last resort.

The DHT entries need to go into a separate ttl cache so they are
invalidated when they get too old.

## Breaking changes

`iroh_dns_server::config::Config` struct has a new field `mainline`.

## Notes & open questions

The issue with async in pkarr has been solved.

Note: caching will be integrated into pkarr in v2. Once that is out we
might be able to remove the cache again and just use pkarr with an
appropriate sized cache configured.

## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [ ] Tests if relevant.

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Co-authored-by: Franz Heinzmann <frando@unbiskant.org>