Add a DevTools extension for riverpod

[kenzieschmoll]

Creating this issue to track the effort of adding a DevTools extension for riverpod, similar to the extension provided for package:provider. @adsonpleal originally pushed up this PR in the DevTools codebase before the DevTools extensions framework was available.

Per @rrousselGit's comment here, it sounds like this is already on his radar, but I couldn't find a tracking issue to track this effort, so I created this one.

[rrousselGit]

There's a tag for tracking it. https://github.com/rrousselGit/riverpod/issues?q=is%3Aopen+is%3Aissue+label%3Adevtool

It is planned but likely won't be coming before the 3.0 release, because I have some specials plans for it.

[kenzieschmoll]

Great, thanks for the link to track!