Update headlamp-plugin and charts to use pluginctl package

[Faakhir30]

Followup from #3455

• Reuse pluginctl in headlamp-plugin
• Use pluginctl for pluginManager in charts
• Update headlamp-plugin readme

Testing

tested using npm link.

1. Create a global symlink:

cd plugins/pluginctl
npm link

2. In the project where you want to use the local version:

npm link @headlamp-k8s/pluginctl

3. Run tests for unpublished headlamp-plugin
cd plugins/headlamp-plugin && ./test-headlamp-plugin.js

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Note: This PR requires #3455 to be merged first and @headlamp-k8s/pluginctl package to be released first.

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[Hazmi35]

I would've prefer to not use npm at all here and provide an image for pluginctl.