Plugins: Extend panel menu with commands from plugins

[mckn]

What is this feature?
This PR adds the functionality for app plugins to add command menu items to the dashboard panel menu.

In PR #63089 we extended that functionality by adding support to add command extensions in the dashboard panel menu.

The difference between links and commands is that links will generate a real href link that will navigate the user away from the current page when clicked. Commands on the other hand will keep the user on the current page but call a command handler function that is provided when registering the plugin.

We do want to refactor and try to make the code a bit simpler but will do that in follow up PRs since it won't affect the external APIs of this feature and we do want to start to dog food this internally.

The example below show case how to extend the dashboard panel menu with a command:

1. First of all you need to enable preload: true for your plugin. We are collecting the extensions during the preload process of a plugin.
2. Secondly you need to configure an extension for your plugin. Example: https://github.com/grafana/grafana-plugin-examples/blob/mckn/plugin-link-tests/examples/app-basic/src/module.ts
3. You can now navigate to a dashboard and enjoy the power of plugin extensions in the panel menu.

Why do we need this feature?
To improve the user experience by making the UI feel less fragmented into different apps rather than one unified experience.

Who is this feature for?
End users and plugin developers

Which issue(s) does this PR fix?:
Related Epic #61658

[mckn]

We are introducing breaking changes compared to main but this feature is not yet released in any version so it won't be an issue.

[jackw]

LGTM! 🚀

Drone doesn't seem to mind but I'm not sure about the commit history. It appears there are commits here that are from #63089 🤔