PSA: Async DNS resolver API in WebRTC is changing

[Harald Alvestrand]

If you don't write code interacting with the DNS through WebRTC, you can stop reading now.

Asynchronous DNS lookups in WebRTC has traditionally been handled by the rtc::AsyncResolver class, firing a signal on resolution completion. This, however, was using sigslot, which is being phased out.

This is being replaced with a newer API that uses callbacks instead, called webrtc::AsyncDnsResolverInterface. Default implementations exist that allow old code to continue to work for now, but those are in the process of being marked [[deprecated]].

The old interfaces will be removed from the codebase no earlier than December 1, 2023, which should roll into Chrome no earlier than Chrome 121.

Tracking bug: https://crbug.com/webrtc/12598

Harald