fix: do not bind a mainline DHT socket

[Frando]

Description

By merging #2188 we unknowingly made iroh bind a socket for a mainline DHT.
Pkarr includes code for querying records from the bittorent mainline DHT behind a dht feature flag, which is disabled in iroh-net but with #2188 is enabled in iroh-dns-server, and due to feature unification in cargo workspaces this silently also enabled the feature in iroh-net.

The DHT is not used because we only use the relay_ methods of the pkarr client in iroh-net. It is binding a socket, but not doing anything else because it has no bootstrap node configured.

Still, this is unfortunate - we are shipping code and binding a socket for a feature we don't (yet) use.

From a cursory skim of the source code, this would still be the same with the recently released pkarr v2.

We should check the docs on feature unification if there is a clean way around that.

For now, this PR makes iroh not bind a socket for an unused mainline DHT client. Instead, we just use reqwest directly, which is the same as what PkarrClient::relay_put would do here.

Breaking Changes

Notes & open questions

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.
• All breaking changes documented.

[flub]

Please use the checklist :)