gcloud backend

[glasser]

This is somewhat discussed in #3709 as a broader concept of plugin backends, but I figured it would be nice to break this out as a potential standalone backend, even just to be potentially rejected.

Concrete issue: in order to use kubectl to interact with Google Cloud Kubernetes clusters, you need to have gke-gcloud-auth-plugin installed, so I would like my company's project repositories to install it via mise along with the rest of our dev environment. This tool is only officially distributed as a "component" of the gcloud CLI, which you install by running a gcloud components install command after installing the CLI.

https://github.com/jthegedus/asdf-gcloud exists and does allow you to provide a file listing components to install with gcloud but it seems more designed for tracking that list at a single global homedir level, not per repository. It would be great to have a more direct way to use mise to ensure that gke-gcloud-auth-plugin is available in a directory.

[glasser]

(it's possible this can be solved fully via hooks — haven't quite got it working but gonna try a bit harder.)

[glasser]

I don't feel like I've gotten this to be solved in a nice enough way via hooks.