Extract secrets manager to new package

[dlqqq]

jupyter_ai_jupyternaut currently serves the secrets manager & its API routes. However, we should make this usable by other extensions & persona packages to allow users to add an authentication mechanism without installing Jupyternaut.

We will want to build a new jupyter_ai_secrets_manager labextension package that also provides the React component used to render the Secrets UI which connects to the Secrets API it serves.