Add a manager API to query available webapp plugins

The webui and control panel need to detect if specific webapp plugin's routes are available or not, to prevent unnecessary `fetch()` to a non-existent URL.

Since there are two classes of webapp plugins, we need to distinguish the publicity level of the query depending whether the user has logged in or not:

- (public) Used during the login process (e.g., TOTP) → default for the existing plugins
- (private) Only used after the login process
Let's update the webapp plugin interface to provide a publicity flag and use it to report the plugin availability to the frontend.

JIRA Issue: BA-183