refactor(iroh-net): small improvements to dns code

[divagant-martian]

Description

• Move helper methods to a trait extension to make them more ergonomic to use.
• Avoid temporary allocations via collect in dns lookups.
• Avoid double iterations on lookup results in the relay.
• Remove unnecessary TryParseIp bound on lookup functions.
• Modify comments about ipv4/ipv6 lookup to clarify why we avoid the lookup
  strategy that would naturally fit this use.

Breaking Changes

• iroh_net::dns::lookup_ipv4_ipv6 is removed in favor of iroh_net::dns::ResolverExt::lookup_ipv4_ipv6

Notes & open questions

Was iroh_net::dns::lookup_ipv4_ipv6 meant to be part of the public api? I don't really see why.

Change checklist

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