Support sending arbitrary messages to pub client

[kevmoo]

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 dart-lang/pub#2165

[sigurdm]

We worry that these messages will be used for annoying purposes such as spamming/advertising.

[kevmoo]

This is NOT for arbitrary messages from package authors. But from the server. Maybe we already have this...

[sigurdm]

Ah - gotcha - we don't have this!

[sigurdm]

We do have specific places to put "discontinued+replaced by" and "retracted". But not a place for an arbitrary message.