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?

首先跟踪 DevTools 如何发现和加载扩展，然后调查在执行 `dart pub global activate` 后，是否可以通过 Dart pub cache 识别全局激活的包。定义这些包中的运行时扩展和静态扩展应如何出现在 DevTools 和嵌入式 IDE 中，并验证全局激活包的端到端工作流。

由索引模型根据 Issue 内容生成。