Support extensions from globally activated pub packages

An example of an extension that fits this use case would be an arbitrary development tool package (like an AI-assisted development experience extension or a tool that interacts with the Dart analysis server). These extensions may be runtime or static extensions.

It would be a bit awkward to require a user to add a dev_dependency to every Dart project where they want to use this DevTools extension, since tools like these are not specific to a Dart / Flutter project, but rather to the general Dart / Flutter development experience. The workflow for an end user would be:

1. `dart pub global activate some_pkg`
2. If `some_pkg` provides a DevTools extension, we should be able to load this in DevTools (and also embedded in the IDE).

@jonasfj @sigmundch is there a way to tell from the pub cache what the globally activated pub packages are?