Add support for multiple level deep navigation structure for app plugins

[anaivanov]

Related Slack discussion: https://raintank-corp.slack.com/archives/C01C4K8DETW/p1715688474530869

Why is this needed

Currently the Grafana navigation only supports "one level" under an app plugin.

Soon there are going to be developed cloud applications that would need 2 levels under them. For example an "Example application" at the root level with this format:

Example application
        Page A 
           Child of Page A
           2nd child of Page A
        Page B
        Page C

What would you like to be added:

Add support for multiple level deep navigation structure for app plugins.
This can mean to add support for a children property for the includes property in plugin.json

Who is this feature for?

This feature is going to be useful and used by app plugins in Grafana.