This unlocks migration from AsyncResolver to AsyncDnsResolver for
clients that implement PacketSocketFactory.
A default implementation is provided, so that clients that implement
CreateAsyncResolver will still see their name resolution work.
Bug: webrtc:12598
Change-Id: If835cbc753712e9f5b4bd3d5805c7f7d2a561ee5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/233500
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35131}
