Support sending arbitrary messages to pub client

Git supports this today. So, for instance, GitHub can send a URL to create a pull request from a new branch that was just pushed.

For the pub site, we could support things like

- Tell user one of their packages has been discontinued
- Future: there is a known issue w/ specific version of a package they are using.

etc etc

See also https://github.com/dart-lang/pub/issues/2165