node-shell: add node shell plugin

[farodin91]

Fixes headlamp-k8s/headlamp#996

Possible Improvements:

• ensure pod is running
• improve error handling
• ensure pod is stopped
• add testing
• Windows Support

[farodin91]

@joaquimrocha Would you like to review?

[joaquimrocha]

I tested it and it works very well! Thanks @farodin91 .
I am still torn between having it as a plugin or having it in the core, but since it shouldn't work in certain environments, maybe it's better as a plugin.

I also realized that we should make it easier for plugins to declare cluster-based settings. These would show up both in the plugin settings, with the cluster selector already automatically done for them, and in the cluster's own settings, as sections.

Since I am leaving on vacation for 2 weeks, I will leave a deeper review of the code to @illume .