Design how to generalize the AI settings

[dlqqq]

Currently, the Jupyternaut settings & the ConfigManager is defined inside of this package. However, other personas will likely need to be able to provide their own Settings UI & backend with their persona.

We will want to design some way for other extensions to do so. I was imagining the following:

1. A single "AI settings" page provided through the menu bar, as done currently with "Jupyternaut settings"
2. Make the page contain multiple tabs, e.g. "Jupyternaut persona", "Secrets", "Tool approval"
3. Allow other extensions to add new tabs to this page through some Lumino plugin system.