Show a warning when a service does not listen on an advertised IPv6 address

*cough* vik.hk

1. This will require a way to communicate between the synchronization job and the `SocketsHttpHandler` it `HttpClient` uses
- `SocketsHttpConnectionContext` has an `HttpRequestMessage InitialRequestMessage` property
3. What should the API look like for storing this?

Start by tracing the synchronization job and the SocketsHttpHandler used by HttpClient, then read SocketsHttpConnectionContext.InitialRequestMessage as the issue's stated communication point. The issue is done only after the API for communicating the IPv6 listening result is agreed and the warning behavior is defined.

Written by the indexing model from the issue text.